Filing Details

SECURITIES AND EXCHANGE COMMISSION Washington, D.C. 20549 ________________ Schedule 13G ________________ INFORMATION TO BE INCLUDED IN STATEMENTS PURSUANT TO RULES 13d-1(b),(c) AND (d) AND AMENDMENTS THERETO FILED PURSUANT TO 13d-2 UNDER THE SECURITIES EXCHANGE ACT OF 1934* United States Steel Corporation (Name of Issuer) Common Stock, par value $1.00 (Title of Class of Securities) 912909108 (CUSIP Number) February 27, 2018 (Date of Event Which Requires Filing of this Statement) Check the appropriate box to designate the rule pursuant to which this Schedule is filed: [ ] Rule 13d-1(b) [X] Rule 13d-1(c) [ ] Rule 13d-1(d) ___________ * The remainder of this cover page shall be filled out for a reporting person's initial filing on this form with respect to the subject class of securities, and for any subsequent amendment containing information which would alter the disclosures provided in a prior cover page. The information required in the remainder of this cover page shall not be deemed to be "filed" for the purpose of Section 18 of the Securities Exchange Act of 1934 ("Act") or otherwise subject to the liabilities of that section of the Act but shall be subject to all other provisions of the Act (however, see the Notes).

Schedule 13G PAGE 2 of 21 CUSIP No. 912909108 _____________________________________________________________________________ (1) NAME OF REPORTING PERSON S.S. OR I.R.S. IDENTIFICATION NO. OF ABOVE PERSON Viking Global Investors LP _____________________________________________________________________________ (2) CHECK THE APPROPRIATE BOX IF A MEMBER OF A GROUP (a) [ ] (b) [x] _____________________________________________________________________________ (3) SEC USE ONLY _____________________________________________________________________________ (4) CITIZENSHIP OR PLACE OF ORGANIZATION Delaware _____________________________________________________________________________ NUMBER OF (5) SOLE VOTING POWER 0 SHARES ______________________________________________________________ BENEFICIALLY (6) SHARED VOTING POWER 9,662,352 OWNED BY ______________________________________________________________ EACH (7) SOLE DISPOSITIVE POWER 0 REPORTING ______________________________________________________________ PERSON WITH (8) SHARED DISPOSITIVE POWER 9,662,352 _____________________________________________________________________________ (9) AGGREGATE AMOUNT BENEFICIALLY OWNED BY EACH REPORTING PERSON 9,662,352 _____________________________________________________________________________ (10) CHECK BOX IF THE AGGREGATE AMOUNT IN ROW (9) EXCLUDES CERTAIN SHARES [ ] _____________________________________________________________________________ (11) PERCENT OF CLASS REPRESENTED BY AMOUNT IN ROW (9) 5.5% _____________________________________________________________________________ (12) TYPE OF REPORTING PERSON PN _____________________________________________________________________________ Schedule 13G PAGE 3 of 21 CUSIP No. 912909108 _____________________________________________________________________________ (1) NAME OF REPORTING PERSON S.S. OR I.R.S. IDENTIFICATION NO. OF ABOVE PERSON Viking Global Performance LLC _____________________________________________________________________________ (2) CHECK THE APPROPRIATE BOX IF A MEMBER OF A GROUP (a) [ ] (b) [x] _____________________________________________________________________________ (3) SEC USE ONLY _____________________________________________________________________________ (4) CITIZENSHIP OR PLACE OF ORGANIZATION Delaware _____________________________________________________________________________ NUMBER OF (5) SOLE VOTING POWER 0 SHARES ______________________________________________________________ BENEFICIALLY (6) SHARED VOTING POWER 6,076,954 OWNED BY ______________________________________________________________ EACH (7) SOLE DISPOSITIVE POWER 0 REPORTING ______________________________________________________________ PERSON WITH (8) SHARED DISPOSITIVE POWER 6,076,954 _____________________________________________________________________________ (9) AGGREGATE AMOUNT BENEFICIALLY OWNED BY EACH REPORTING PERSON 6,076,954 _____________________________________________________________________________ (10) CHECK BOX IF THE AGGREGATE AMOUNT IN ROW (9) EXCLUDES CERTAIN SHARES [ ] _____________________________________________________________________________ (11) PERCENT OF CLASS REPRESENTED BY AMOUNT IN ROW (9) 3.5% _____________________________________________________________________________ (12) TYPE OF REPORTING PERSON OO _____________________________________________________________________________ Schedule 13G PAGE 4 of 21 CUSIP No. 912909108 _____________________________________________________________________________ (1) NAME OF REPORTING PERSON S.S. OR I.R.S. IDENTIFICATION NO. OF ABOVE PERSON Viking Global Equities LP _____________________________________________________________________________ (2) CHECK THE APPROPRIATE BOX IF A MEMBER OF A GROUP (a) [ ] (b) [x] _____________________________________________________________________________ (3) SEC USE ONLY _____________________________________________________________________________ (4) CITIZENSHIP OR PLACE OF ORGANIZATION Delaware _____________________________________________________________________________ NUMBER OF (5) SOLE VOTING POWER 0 SHARES ______________________________________________________________ BENEFICIALLY (6) SHARED VOTING POWER 2,072,247 OWNED BY ______________________________________________________________ EACH (7) SOLE DISPOSITIVE POWER 0 REPORTING ______________________________________________________________ PERSON WITH (8) SHARED DISPOSITIVE POWER 2,072,247 _____________________________________________________________________________ (9) AGGREGATE AMOUNT BENEFICIALLY OWNED BY EACH REPORTING PERSON 2,072,247 _____________________________________________________________________________ (10) CHECK BOX IF THE AGGREGATE AMOUNT IN ROW (9) EXCLUDES CERTAIN SHARES [ ] _____________________________________________________________________________ (11) PERCENT OF CLASS REPRESENTED BY AMOUNT IN ROW (9) 1.2% _____________________________________________________________________________ (12) TYPE OF REPORTING PERSON PN _____________________________________________________________________________ Schedule 13G PAGE 5 of 21 CUSIP No. 912909108 _____________________________________________________________________________ (1) NAME OF REPORTING PERSON S.S. OR I.R.S. IDENTIFICATION NO. OF ABOVE PERSON Viking Global Equities II LP _____________________________________________________________________________ (2) CHECK THE APPROPRIATE BOX IF A MEMBER OF A GROUP (a) [ ] (b) [x] _____________________________________________________________________________ (3) SEC USE ONLY _____________________________________________________________________________ (4) CITIZENSHIP OR PLACE OF ORGANIZATION Delaware _____________________________________________________________________________ NUMBER OF (5) SOLE VOTING POWER 0 SHARES ______________________________________________________________ BENEFICIALLY (6) SHARED VOTING POWER 121,534 OWNED BY ______________________________________________________________ EACH (7) SOLE DISPOSITIVE POWER 0 REPORTING ______________________________________________________________ PERSON WITH (8) SHARED DISPOSITIVE POWER 121,534 _____________________________________________________________________________ (9) AGGREGATE AMOUNT BENEFICIALLY OWNED BY EACH REPORTING PERSON 121,534 _____________________________________________________________________________ (10) CHECK BOX IF THE AGGREGATE AMOUNT IN ROW (9) EXCLUDES CERTAIN SHARES [ ] _____________________________________________________________________________ (11) PERCENT OF CLASS REPRESENTED BY AMOUNT IN ROW (9) 0.1% _____________________________________________________________________________ (12) TYPE OF REPORTING PERSON PN _____________________________________________________________________________ Schedule 13 PAGE 6 of 21 CUSIP No. 912909108 _____________________________________________________________________________ (1) NAME OF REPORTING PERSON S.S. OR I.R.S. IDENTIFICATION NO. OF ABOVE PERSON VGE III Portfolio Ltd. _____________________________________________________________________________ (2) CHECK THE APPROPRIATE BOX IF A MEMBER OF A GROUP (a) [ ] (b) [x] _____________________________________________________________________________ (3) SEC USE ONLY _____________________________________________________________________________ (4) CITIZENSHIP OR PLACE OF ORGANIZATION Cayman Islands _____________________________________________________________________________ NUMBER OF (5) SOLE VOTING POWER 0 SHARES ______________________________________________________________ BENEFICIALLY (6) SHARED VOTING POWER 3,883,173 OWNED BY ______________________________________________________________ EACH (7) SOLE DISPOSITIVE POWER 0 REPORTING ______________________________________________________________ PERSON WITH (8) SHARED DISPOSITIVE POWER 3,883,173 _____________________________________________________________________________ (9) AGGREGATE AMOUNT BENEFICIALLY OWNED BY EACH REPORTING PERSON 3,883,173 _____________________________________________________________________________ (10) CHECK BOX IF THE AGGREGATE AMOUNT IN ROW (9) EXCLUDES CERTAIN SHARES [ ] _____________________________________________________________________________ (11) PERCENT OF CLASS REPRESENTED BY AMOUNT IN ROW (9) 2.2% _____________________________________________________________________________ (12) TYPE OF REPORTING PERSON CO _____________________________________________________________________________ Schedule 13G PAGE 7 of 21 CUSIP No. 912909108 _____________________________________________________________________________ (1) NAME OF REPORTING PERSON S.S. OR I.R.S. IDENTIFICATION NO. OF ABOVE PERSON Viking Long Fund GP LLC _____________________________________________________________________________ (2) CHECK THE APPROPRIATE BOX IF A MEMBER OF A GROUP (a) [ ] (b) [x] _____________________________________________________________________________ (3) SEC USE ONLY _____________________________________________________________________________ (4) CITIZENSHIP OR PLACE OF ORGANIZATION Delaware _____________________________________________________________________________ NUMBER OF (5) SOLE VOTING POWER 0 SHARES ______________________________________________________________ BENEFICIALLY (6) SHARED VOTING POWER 2,734,625 OWNED BY ______________________________________________________________ EACH (7) SOLE DISPOSITIVE POWER 0 REPORTING ______________________________________________________________ PERSON WITH (8) SHARED DISPOSITIVE POWER 2,734,625 _____________________________________________________________________________ (9) AGGREGATE AMOUNT BENEFICIALLY OWNED BY EACH REPORTING PERSON 2,734,625 _____________________________________________________________________________ (10) CHECK BOX IF THE AGGREGATE AMOUNT IN ROW (9) EXCLUDES CERTAIN SHARES [ ] _____________________________________________________________________________ (11) PERCENT OF CLASS REPRESENTED BY AMOUNT IN ROW (9) 1.6% _____________________________________________________________________________ (12) TYPE OF REPORTING PERSON OO _____________________________________________________________________________ Schedule 13G PAGE 8 of 21 CUSIP No. 912909108 _____________________________________________________________________________ (1) NAME OF REPORTING PERSON S.S. OR I.R.S. IDENTIFICATION NO. OF ABOVE PERSON Viking Long Fund Master Ltd. _____________________________________________________________________________ (2) CHECK THE APPROPRIATE BOX IF A MEMBER OF A GROUP (a) [ ] (b) [x] _____________________________________________________________________________ (3) SEC USE ONLY _____________________________________________________________________________ (4) CITIZENSHIP OR PLACE OF ORGANIZATION Cayman Islands _____________________________________________________________________________ NUMBER OF (5) SOLE VOTING POWER 0 SHARES ______________________________________________________________ BENEFICIALLY (6) SHARED VOTING POWER 2,734,625 OWNED BY ______________________________________________________________ EACH (7) SOLE DISPOSITIVE POWER 0 REPORTING ______________________________________________________________ PERSON WITH (8) SHARED DISPOSITIVE POWER 2,734,625 _____________________________________________________________________________ (9) AGGREGATE AMOUNT BENEFICIALLY OWNED BY EACH REPORTING PERSON 2,734,625 _____________________________________________________________________________ (10) CHECK BOX IF THE AGGREGATE AMOUNT IN ROW (9) EXCLUDES CERTAIN SHARES [ ] _____________________________________________________________________________ (11) PERCENT OF CLASS REPRESENTED BY AMOUNT IN ROW (9) 1.6% _____________________________________________________________________________ (12) TYPE OF REPORTING PERSON CO _____________________________________________________________________________ Schedule 13G PAGE 9 of 21 CUSIP No. 912909108 _____________________________________________________________________________ (1) NAME OF REPORTING PERSON S.S. OR I.R.S. IDENTIFICATION NO. OF ABOVE PERSON Viking Global Opportunities GP LLC _____________________________________________________________________________ (2) CHECK THE APPROPRIATE BOX IF A MEMBER OF A GROUP (a) [ ] (b) [x] _____________________________________________________________________________ (3) SEC USE ONLY _____________________________________________________________________________ (4) CITIZENSHIP OR PLACE OF ORGANIZATION Delaware _____________________________________________________________________________ NUMBER OF (5) SOLE VOTING POWER 0 SHARES ______________________________________________________________ BENEFICIALLY (6) SHARED VOTING POWER 850,773 OWNED BY ______________________________________________________________ EACH (7) SOLE DISPOSITIVE POWER 0 REPORTING ______________________________________________________________ PERSON WITH (8) SHARED DISPOSITIVE POWER 850,773 _____________________________________________________________________________ (9) AGGREGATE AMOUNT BENEFICIALLY OWNED BY EACH REPORTING PERSON 850,773 _____________________________________________________________________________ (10) CHECK BOX IF THE AGGREGATE AMOUNT IN ROW (9) EXCLUDES CERTAIN SHARES [ ] _____________________________________________________________________________ (11) PERCENT OF CLASS REPRESENTED BY AMOUNT IN ROW (9) 0.5% _____________________________________________________________________________ (12) TYPE OF REPORTING PERSON OO _____________________________________________________________________________ Schedule 13G PAGE 10 of 21 CUSIP No. 912909108 _____________________________________________________________________________ (1) NAME OF REPORTING PERSON S.S. OR I.R.S. IDENTIFICATION NO. OF ABOVE PERSON Viking Global Opportunities Portfolio GP LLC _____________________________________________________________________________ (2) CHECK THE APPROPRIATE BOX IF A MEMBER OF A GROUP (a) [ ] (b) [x] _____________________________________________________________________________ (3) SEC USE ONLY _____________________________________________________________________________ (4) CITIZENSHIP OR PLACE OF ORGANIZATION Delaware _____________________________________________________________________________ NUMBER OF (5) SOLE VOTING POWER 0 SHARES ______________________________________________________________ BENEFICIALLY (6) SHARED VOTING POWER 850,773 OWNED BY ______________________________________________________________ EACH (7) SOLE DISPOSITIVE POWER 0 REPORTING ______________________________________________________________ PERSON WITH (8) SHARED DISPOSITIVE POWER 850,773 _____________________________________________________________________________ (9) AGGREGATE AMOUNT BENEFICIALLY OWNED BY EACH REPORTING PERSON 850,773 _____________________________________________________________________________ (10) CHECK BOX IF THE AGGREGATE AMOUNT IN ROW (9) EXCLUDES CERTAIN SHARES [ ] _____________________________________________________________________________ (11) PERCENT OF CLASS REPRESENTED BY AMOUNT IN ROW (9) 0.5% _____________________________________________________________________________ (12) TYPE OF REPORTING PERSON OO _____________________________________________________________________________ Schedule 13G PAGE 11 of 21 CUSIP No. 912909108 _____________________________________________________________________________ (1) NAME OF REPORTING PERSON S.S. OR I.R.S. IDENTIFICATION NO. OF ABOVE PERSON Viking Global Opportunities Liquid Portfolio Sub-Master LP _____________________________________________________________________________ (2) CHECK THE APPROPRIATE BOX IF A MEMBER OF A GROUP (a) [ ] (b) [x] _____________________________________________________________________________ (3) SEC USE ONLY _____________________________________________________________________________ (4) CITIZENSHIP OR PLACE OF ORGANIZATION Cayman Islands _____________________________________________________________________________ NUMBER OF (5) SOLE VOTING POWER 0 SHARES ______________________________________________________________ BENEFICIALLY (6) SHARED VOTING POWER 850,773 OWNED BY ______________________________________________________________ EACH (7) SOLE DISPOSITIVE POWER 0 REPORTING ______________________________________________________________ PERSON WITH (8) SHARED DISPOSITIVE POWER 850,773 _____________________________________________________________________________ (9) AGGREGATE AMOUNT BENEFICIALLY OWNED BY EACH REPORTING PERSON 850,773 _____________________________________________________________________________ (10) CHECK BOX IF THE AGGREGATE AMOUNT IN ROW (9) EXCLUDES CERTAIN SHARES [ ] _____________________________________________________________________________ (11) PERCENT OF CLASS REPRESENTED BY AMOUNT IN ROW (9) 0.5% _____________________________________________________________________________ (12) TYPE OF REPORTING PERSON PN _____________________________________________________________________________ Schedule 13G PAGE 12 of 21 CUSIP No. 912909108 _____________________________________________________________________________ (1) NAME OF REPORTING PERSON S.S. OR I.R.S. IDENTIFICATION NO. OF ABOVE PERSON O. Andreas Halvorsen _____________________________________________________________________________ (2) CHECK THE APPROPRIATE BOX IF A MEMBER OF A GROUP (a) [ ] (b) [x] _____________________________________________________________________________ (3) SEC USE ONLY _____________________________________________________________________________ (4) CITIZENSHIP OR PLACE OF ORGANIZATION Norway _____________________________________________________________________________ NUMBER OF (5) SOLE VOTING POWER 0 SHARES ______________________________________________________________ BENEFICIALLY (6) SHARED VOTING POWER 9,662,352 OWNED BY ______________________________________________________________ EACH (7) SOLE DISPOSITIVE POWER 0 REPORTING ______________________________________________________________ PERSON WITH (8) SHARED DISPOSITIVE POWER 9,662,352 _____________________________________________________________________________ (9) AGGREGATE AMOUNT BENEFICIALLY OWNED BY EACH REPORTING PERSON 9,662,352 _____________________________________________________________________________ (10) CHECK BOX IF THE AGGREGATE AMOUNT IN ROW (9) EXCLUDES CERTAIN SHARES [ ] _____________________________________________________________________________ (11) PERCENT OF CLASS REPRESENTED BY AMOUNT IN ROW (9) 5.5% _____________________________________________________________________________ (12) TYPE OF REPORTING PERSON IN _____________________________________________________________________________ Schedule 13G PAGE 13 of 21 CUSIP No. 912909108 _____________________________________________________________________________ (1) NAME OF REPORTING PERSON S.S. OR I.R.S. IDENTIFICATION NO. OF ABOVE PERSON David C. Ott _____________________________________________________________________________ (2) CHECK THE APPROPRIATE BOX IF A MEMBER OF A GROUP (a) [ ] (b) [x] _____________________________________________________________________________ (3) SEC USE ONLY _____________________________________________________________________________ (4) CITIZENSHIP OR PLACE OF ORGANIZATION United States _____________________________________________________________________________ NUMBER OF (5) SOLE VOTING POWER 0 SHARES ______________________________________________________________ BENEFICIALLY (6) SHARED VOTING POWER 9,662,352 OWNED BY ______________________________________________________________ EACH (7) SOLE DISPOSITIVE POWER 0 REPORTING ______________________________________________________________ PERSON WITH (8) SHARED DISPOSITIVE POWER 9,662,352 _____________________________________________________________________________ (9) AGGREGATE AMOUNT BENEFICIALLY OWNED BY EACH REPORTING PERSON 9,662,352 _____________________________________________________________________________ (10) CHECK BOX IF THE AGGREGATE AMOUNT IN ROW (9) EXCLUDES CERTAIN SHARES [ ] _____________________________________________________________________________ (11) PERCENT OF CLASS REPRESENTED BY AMOUNT IN ROW (9) 5.5% _____________________________________________________________________________ (12) TYPE OF REPORTING PERSON IN _____________________________________________________________________________ Schedule 13G PAGE 14 of 21 CUSIP No. 912909108 _____________________________________________________________________________ (1) NAME OF REPORTING PERSON S.S. OR I.R.S. IDENTIFICATION NO. OF ABOVE PERSON Rose S. Shabet _____________________________________________________________________________ (2) CHECK THE APPROPRIATE BOX IF A MEMBER OF A GROUP (a) [ ] (b) [x] _____________________________________________________________________________ (3) SEC USE ONLY _____________________________________________________________________________ (4) CITIZENSHIP OR PLACE OF ORGANIZATION United States _____________________________________________________________________________ NUMBER OF (5) SOLE VOTING POWER 0 SHARES ______________________________________________________________ BENEFICIALLY (6) SHARED VOTING POWER 9,662,352 OWNED BY ______________________________________________________________ EACH (7) SOLE DISPOSITIVE POWER 0 REPORTING ______________________________________________________________ PERSON WITH (8) SHARED DISPOSITIVE POWER 9,662,352 _____________________________________________________________________________ (9) AGGREGATE AMOUNT BENEFICIALLY OWNED BY EACH REPORTING PERSON 9,662,352 _____________________________________________________________________________ (10) CHECK BOX IF THE AGGREGATE AMOUNT IN ROW (9) EXCLUDES CERTAIN SHARES [ ] _____________________________________________________________________________ (11) PERCENT OF CLASS REPRESENTED BY AMOUNT IN ROW (9) 5.5% _____________________________________________________________________________ (12) TYPE OF REPORTING PERSON IN _____________________________________________________________________________ Schedule 13G PAGE 15 of 21 CUSIP No. 912909108 ITEM 1(a). NAME OF ISSUER: United States Steel Corporation ITEM 1(b). ADDRESS OF ISSUER'S PRINCIPAL EXECUTIVE OFFICES: 600 Grant Street, Room 1500 Pittsburgh, Pennsylvania 15219 ITEM 2(a). NAME OF PERSON FILING: Viking Global Investors LP ("VGI"), Viking Global Performance LLC ("VGP"), Viking Global Equities LP ("VGE"), Viking Global Equities II LP ("VGEII"), VGE III Portfolio Ltd. ("VGEIII"), Viking Long Fund GP LLC ("VLFGP"), Viking Long Fund Master Ltd. ("VLFM"), Viking Global Opportunities GP LLC ("Opportunities GP"), Viking Global Opportunities Portfolio GP LLC ("Opportunities Portfolio GP"), Viking Global Opportunities Liquid Portfolio Sub-Master LP ("Opportunities Fund"), O. Andreas Halvorsen, David C. Ott and Rose S. Shabet (collectively, the "Reporting Persons") ITEM 2(b). ADDRESS OF PRINCIPAL OFFICE OR, IF NONE, RESIDENCE: The business address of each of the Reporting Persons is 55 Railroad Avenue, Greenwich, Connecticut 06830. ITEM 2(c). CITIZENSHIP: VGI, VGE and VGEII are Delaware limited partnerships; VGEIII and VLFM are Cayman Islands exempted companies; VGP, VLFGP, Opportunities GP and Opportunities Portfolio GP are Delaware limited liability companies; and Opportunities Fund is a Cayman Islands exempted limited partnership. O. Andreas Halvorsen is a citizen of Norway. David C. Ott and Rose S. Shabet are citizens of the United States. ITEM 2(d). TITLE OF CLASS OF SECURITIES: Common Stock, par value $1.00 ("Common Stock") ITEM 2(e). CUSIP NUMBER: 912909108 ITEM 3. IF THIS STATEMENT IS FILED PURSUANT TO 13d-1(b) OR 13d-2(b) OR (c), CHECK WHETHER THE PERSON FILING IS A: (a) [ ] Broker or dealer registered under Section 15 of the Act (b) [ ] Bank as defined in Section 3(a)(6) of the Act (c) [ ] Insurance Company as defined in Section 3(a)(19) of the Act (d) [ ] Investment Company registered under Section 8 of the Investment Company Act of 1940 (e) [ ] Investment Adviser registered under Section 203 of the Investment Advisers Act of 1940: see Rule 13d- 1(b)(1)(ii)(E) (f) [ ] Employee Benefit Plan, Pension Fund which is subject to the provisions of the Employee Retirement Income Security Act of 1974 or Endowment Fund; see Rule 13d- 1(b)(1)(ii)(F) Schedule 13G PAGE 16 of 21 CUSIP No. 912909108 (g) [ ] Parent Holding Company, in accordance with Rule 13d- 1(b)(ii)(G) (h) [ ] Savings Associations as defined in Section 3(b) of the Federal Deposit Insurance Act (i) [ ] Church Plan that is excluded from the definition of an investment company under Section 3(c)(14) of the Investment Company Act of 1940 (j) [ ] A non-U.S. institution in accordance with Rule 240.13d-1(b)(1)(ii)(J) (k) [ ] Group, in accordance with Rule 240.13d-1(b)(1)(ii)(K). If filing as a non-U.S. institution in accordance with Rule 240.13d-1(b)(1)(ii)(J), please specify the type of institution: ITEM 4. OWNERSHIP. A. VGI (a) Amount beneficially owned: 9,662,352 (b) Percent of class: 5.5% (c) Number of shares as to which such person has: (i) sole power to vote or to direct the vote 0 (ii) shared power to vote or to direct the vote 9,662,352 (iii) sole power to dispose or to direct the disposition of 0 (iv) shared power to dispose or to direct the disposition of 9,662,352 VGI provides managerial services to VGE, VGEII, VGEIII, VLFM and Opportunities Fund. VGI has the authority to dispose of and vote the shares of Common Stock. Based on Rule 13d-3 of the Securities Exchange Act of 1934, as amended (the "Act"), VGI may be deemed to beneficially own the shares of Common Stock directly held by VGE, VGEII, VGEIII, VLFM and Opportunities Fund. VGI does not directly own any shares of Common Stock. Schedule 13G PAGE 17 of 21 CUSIP No. 912909108 B. VGP (a) Amount beneficially owned: 6,076,954 (b) Percent of class: 3.5% (c) Number of shares as to which such person has: (i) sole power to vote or to direct the vote 0 (ii) shared power to vote or to direct the vote 6,076,954 (iii) sole power to dispose or to direct the disposition of 0 (iv) shared power to dispose or to direct the disposition of 6,076,954 VGP, as the general partner of VGE and VGEII, has the authority to dispose of and vote the shares of Common Stock directly owned by VGE and VGEII. VGP serves as investment manager to VGEIII and has the authority to dispose of and vote the shares of Common Stock directly owned by VGEIII. VGP does not directly own any shares of Common Stock. Based on Rule 13d-3 of the Act, VGP may be deemed to beneficially own the shares of Common Stock directly held by VGE, VGEII and VGEIII. C. VLFGP (a) Amount beneficially owned: 2,734,625 (b) Percent of class: 1.6% (c) Number of shares as to which such person has: (i) sole power to vote or to direct the vote 0 (ii) shared power to vote or to direct the vote 2,734,625 (iii) sole power to dispose or to direct the disposition of 0 (iv) shared power to dispose or to direct the disposition of 2,734,625 VLFGP serves as the investment manager of VLFM and has the authority to dispose of and vote the shares of Common Stock directly owned by VLFM. VLFGP does not directly own any shares of Common Stock. Based on Rule 13d-3 of the Act, VLFGP may be deemed to beneficially own the shares of Common Stock directly held by VLFM. D. Opportunities GP (a) Amount beneficially owned: 850,773 (b) Percent of class: 0.5% (c) Number of shares as to which such person has: (i) sole power to vote or to direct the vote 0 (ii) shared power to vote or to direct the vote 850,773 (iii) sole power to dispose or to direct the disposition of 0 (iv) shared power to dispose or to direct the disposition of 850,773 Opportunities GP serves as the sole member of Opportunities Portfolio GP and has the authority to dispose of and vote the shares of Common Stock controlled by Opportunities Portfolio GP, which consists of the shares of Common Stock directly held by Opportunities Fund. Opportunities GP does not directly own any shares of Common Stock. Based on Rule 13d-3 of the Act, Opportunities GP may be deemed to beneficially own the shares of Common Stock controlled by Opportunities Portfolio GP, which consists of the shares of Common Stock directly held by Opportunities Fund. E. Opportunities Portfolio GP (a) Amount beneficially owned: 850,773 (b) Percent of class: 0.5% (c) Number of shares as to which such person has: (i) sole power to vote or to direct the vote 0 (ii) shared power to vote or to direct the vote 850,773 (iii) sole power to dispose or to direct the disposition of 0 (iv) shared power to dispose or to direct the disposition of 850,773 Opportunities Portfolio GP serves as the general partner of Opportunities Fund and has the authority to dispose of and vote the shares of Common Stock directly owned by Opportunities Fund. Opportunities Portfolio GP does not directly own any shares of Common Stock. Based on Rule 13d-3 of the Act, Opportunities Portfolio GP may be deemed to beneficially own the shares of Common Stock directly held by Opportunities Fund. Schedule 13G PAGE 18 of 21 CUSIP No. 912909108 F. VGE (a) Amount beneficially owned: 2,072,247 (b) Percent of class: 1.2% (c) Number of shares as to which such person has: (i) sole power to vote or to direct the vote 0 (ii) shared power to vote or to direct the vote 2,072,247 (iii) sole power to dispose or to direct the disposition of 0 (iv) shared power to dispose or to direct the disposition of 2,072,247 VGE has the authority to dispose of and vote the shares of Common Stock directly owned by it, which power may be exercised by its general partner, VGP, and by VGI, an affiliate of VGP, which provides managerial services to VGE. G. VGEII (a) Amount beneficially owned: 121,534 (b) Percent of class: 0.1% (c) Number of shares as to which such person has: (i) sole power to vote or to direct the vote 0 (ii) shared power to vote or to direct the vote 121,534 (iii) sole power to dispose or to direct the disposition of 0 (iv) shared power to dispose or to direct the disposition of 121,534 VGEII has the authority to dispose of and vote the shares of Common Stock directly owned by it, which power may be exercised by its general partner, VGP, and by VGI, an affiliate of VGP, which provides managerial services to VGEII. H. VGEIII (a) Amount beneficially owned: 3,883,173 (b) Percent of class: 2.2% (c) Number of shares as to which such person has: (i) sole power to vote or to direct the vote 0 (ii) shared power to vote or to direct the vote 3,883,173 (iii) sole power to dispose or to direct the disposition of 0 (iv) shared power to dispose or to direct the disposition of 3,883,173 VGEIII has the authority to dispose of and vote the shares of Common Stock directly owned by it, which power may be exercised by its investment manager, VGP, and by VGI, an affiliate of VGP, which provides managerial services to VGEIII. Viking Global Equities III Ltd. (a Cayman Islands exempted company) invests substantially all of its assets through VGEIII. I. VLFM (a) Amount beneficially owned: 2,734,625 (b) Percent of class: 1.6% (c) Number of shares as to which such person has: (i) sole power to vote or to direct the vote 0 (ii) shared power to vote or to direct the vote 2,734,625 (iii) sole power to dispose or to direct the disposition of 0 (iv) shared power to dispose or to direct the disposition of 2,734,625 VLFM has the authority to dispose of and vote the shares of Common Stock directly owned by it, which power may be exercised by its investment manager, VLFGP, and by VGI, an affiliate of VLFGP, which provides managerial services to VLFM. Viking Long Fund LP (a Delaware limited partnership) and Viking Long Fund III Ltd. (a Cayman Islands exempted company), through its investment in Viking Long Fund Intermediate LP (a Cayman Islands limited partnership), invest substantially all of their assets through VLFM. J. Opportunities Fund (a) Amount beneficially owned: 850,773 (b) Percent of class: 0.5% (c) Number of shares as to which such person has: (i) sole power to vote or to direct the vote 0 (ii) shared power to vote or to direct the vote 850,773 (iii) sole power to dispose or to direct the disposition of 0 (iv) shared power to dispose or to direct the disposition of 850,773 Opportunities Fund has the authority to dispose of and vote the shares of Common Stock directly owned by it, which power may be exercised by its general partner, Opportunities Portfolio GP, and by VGI, an affiliate of Opportunities Portfolio GP, which provides managerial services to Opportunities Fund. Viking Global Opportunities LP (a Delaware limited partnership) and Viking Global Opportunities III LP (a Cayman Islands exempted limited partnership), through its investment in Viking Global Opportunities Intermediate LP (a Cayman Islands exempted limited partnership), invest substantially all of their assets in Viking Global Opportunities Master LP (a Cayman Islands exempted limited partnership), which in turn invests through Opportunities Fund. Schedule 13G PAGE 19 of 21 CUSIP No. 912909108 K. O. Andreas Halvorsen, David C. Ott and Rose S. Shabet (a) Amount beneficially owned: 9,662,352 (b) Percent of class: 5.5% (c) Number of shares as to which such person has: (i) sole power to vote or to direct the vote 0 (ii) shared power to vote or to direct the vote 9,662,352 (iii) sole power to dispose or to direct the disposition of 0 (iv) shared power to dispose or to direct the disposition of 9,662,352 Mr. Halvorsen, Mr. Ott and Ms. Shabet, as Executive Committee Members of Viking Global Partners LLC, general partner of VGI ("VGPL"), VGP, VLFGP and Opportunities GP, have shared authority to dispose of and vote the shares of Common Stock beneficially owned by VGI, VGP, VLFGP and Opportunities GP. None of Mr. Halvorsen, Mr. Ott and Ms. Shabet directly owns any shares of Common Stock. Based on Rule 13d-3 of the Act, each may be deemed to beneficially own the shares of Common Stock directly held by VGE, VGE II, VGEIII, VLFM and Opportunities Fund. ITEM 5. OWNERSHIP OF FIVE PERCENT OR LESS OF A CLASS. If this statement is being filed to report the fact that as of the date hereof the Reporting Persons has ceased to be the beneficial owner of more than five percent of the class of securities, check the following.[] ITEM 6. OWNERSHIP OF MORE THAN FIVE PERCENT ON BEHALF OF ANOTHER PERSON. Yes, see Item 4. ITEM 7. IDENTIFICATION AND CLASSIFICATION OF THE SUBSIDIARY WHICH ACQUIRED THE SECURITY BEING REPORTED ON BY THE PARENT HOLDING COMPANY. Not applicable. ITEM 8. IDENTIFICATION AND CLASSIFICATION OF MEMBERS OF THE GROUP. Not applicable. ITEM 9. NOTICE OF DISSOLUTION OF GROUP. Not applicable. ITEM 10. CERTIFICATION. (if filing pursuant to Rule 13d-1(c)) By signing below each Reporting Person certifies that, to the best of its knowledge and belief, the securities referred to above were not acquired and are not held for the purpose of or with the effect of changing or influencing the control of the issuer of the securities and were not acquired and not held in connection with or as a participant in any transaction having that purpose or effect. Schedule 13G PAGE 20 of 21 CUSIP No. 912909108 SIGNATURES After reasonable inquiry and to the best of our knowledge and belief, the undersigned certify that the information set forth in this statement is true, complete and correct. DATED: March 9, 2018 /s/ O. ANDREAS HALVORSEN By: O. Andreas Halvorsen - individually and as an Executive Committee Member of VIKING GLOBAL PERFORMANCE LLC, on behalf of itself and VIKING GLOBAL EQUITIES LP, VIKING GLOBAL EQUITIES II LP and VGE III PORTFOLIO LTD., and as an Executive Committee Member of Viking Global Partners LLC, on behalf of VIKING GLOBAL INVESTORS LP, and as an Executive Committee Member of VIKING LONG FUND GP LLC, on behalf of itself and VIKING LONG FUND MASTER LTD., and as an Executive Committee Member of VIKING GLOBAL OPPORTUNITIES GP LLC, on behalf of itself and VIKING GLOBAL OPPORTUNITIES PORTFOLIO GP LLC and VIKING GLOBAL OPPORTUNITIES LIQUID PORTFOLIO SUB-MASTER LP /s/ DAVID C. OTT By: David C. Ott - individually and as an Executive Committee Member of VIKING GLOBAL PERFORMANCE LLC, on behalf of itself and VIKING GLOBAL EQUITIES LP, VIKING GLOBAL EQUITIES II LP and VGE III PORTFOLIO LTD., and as an Executive Committee Member of Viking Global Partners LLC, on behalf of VIKING GLOBAL INVESTORS LP, and as an Executive Committee Member of VIKING LONG FUND GP LLC, on behalf of itself and VIKING LONG FUND MASTER LTD., and as an Executive Committee Member of VIKING GLOBAL OPPORTUNITIES GP LLC, on behalf of itself and VIKING GLOBAL OPPORTUNITIES PORTFOLIO GP LLC and VIKING GLOBAL OPPORTUNITIES LIQUID PORTFOLIO SUB-MASTER LP /s/ ROSE S. SHABET By: Rose S. Shabet - individually and as an Executive Committee Member of VIKING GLOBAL PERFORMANCE LLC, on behalf of itself and VIKING GLOBAL EQUITIES LP, VIKING GLOBAL EQUITIES II LP and VGE III PORTFOLIO LTD., and as an Executive Committee Member of Viking Global Partners LLC, on behalf of VIKING GLOBAL INVESTORS LP, and as an Executive Committee Member of VIKING LONG FUND GP LLC, on behalf of itself and VIKING LONG FUND MASTER LTD., and as an Executive Committee Member of VIKING GLOBAL OPPORTUNITIES GP LLC, on behalf of itself and VIKING GLOBAL OPPORTUNITIES PORTFOLIO GP LLC and VIKING GLOBAL OPPORTUNITIES LIQUID PORTFOLIO SUB-MASTER LP Schedule 13G PAGE 21 of 21 CUSIP No. 912909108 EXHIBIT A - JOINT FILING AGREEMENT This joint filing agreement is made and entered into as of this 9th day of March, 2018, by and among Viking Global Investors LP, Viking Global Performance LLC, Viking Global Equities LP, Viking Global Equities II LP, VGE III Portfolio Ltd., Viking Long Fund GP LLC, Viking Long Fund Master Ltd., Viking Global Opportunities GP LLC, Viking Global Opportunities Portfolio GP LLC, Viking Global Opportunities Liquid Portfolio Sub-Master LP, O. Andreas Halvorsen, David C. Ott and Rose S. Shabet. The parties hereby agree to jointly prepare and file a Schedule 13G with respect to United States Steel Corporation, as well as any amendments thereto, pursuant to the Securities Exchange Act of 1934, as amended. IN WITNESS WHEREOF, the parties hereto have executed this agreement as of the date first set forth above. Dated: March 9, 2018 /s/ O. ANDREAS HALVORSEN By: O. Andreas Halvorsen - individually and as an Executive Committee Member of VIKING GLOBAL PERFORMANCE LLC, on behalf of itself and VIKING GLOBAL EQUITIES LP, VIKING GLOBAL EQUITIES II LP and VGE III PORTFOLIO LTD., and as an Executive Committee Member of Viking Global Partners LLC, on behalf of VIKING GLOBAL INVESTORS LP, and as an Executive Committee Member of VIKING LONG FUND GP LLC, on behalf of itself and VIKING LONG FUND MASTER LTD., and as an Executive Committee Member of VIKING GLOBAL OPPORTUNITIES GP LLC, on behalf of itself and VIKING GLOBAL OPPORTUNITIES PORTFOLIO GP LLC and VIKING GLOBAL OPPORTUNITIES LIQUID PORTFOLIO SUB-MASTER LP /s/ DAVID C. OTT By: David C. Ott - individually and as an Executive Committee Member of VIKING GLOBAL PERFORMANCE LLC, on behalf of itself and VIKING GLOBAL EQUITIES LP, VIKING GLOBAL EQUITIES II LP and VGE III PORTFOLIO LTD., and as an Executive Committee Member of Viking Global Partners LLC, on behalf of VIKING GLOBAL INVESTORS LP, and as an Executive Committee Member of VIKING LONG FUND GP LLC, on behalf of itself and VIKING LONG FUND MASTER LTD., and as an Executive Committee Member of VIKING GLOBAL OPPORTUNITIES GP LLC, on behalf of itself and VIKING GLOBAL OPPORTUNITIES PORTFOLIO GP LLC and VIKING GLOBAL OPPORTUNITIES LIQUID PORTFOLIO SUB-MASTER LP /s/ ROSE S. SHABET By: Rose S. Shabet - individually and as an Executive Committee Member of VIKING GLOBAL PERFORMANCE LLC, on behalf of itself and VIKING GLOBAL EQUITIES LP, VIKING GLOBAL EQUITIES II LP and VGE III PORTFOLIO LTD., and as an Executive Committee Member of Viking Global Partners LLC, on behalf of VIKING GLOBAL INVESTORS LP, and as an Executive Committee Member of VIKING LONG FUND GP LLC, on behalf of itself and VIKING LONG FUND MASTER LTD., and as an Executive Committee Member of VIKING GLOBAL OPPORTUNITIES GP LLC, on behalf of itself and VIKING GLOBAL OPPORTUNITIES PORTFOLIO GP LLC and VIKING GLOBAL OPPORTUNITIES LIQUID PORTFOLIO SUB-MASTER LP