13D Filing: Baker Bros. Advisors and Mirati Therapeutics Inc. (MRTX)

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CUSIP No.           60468T105

1.

NAMES OF REPORTING PERSONS

Baker Bros. Advisors LP

2.

CHECK THE APPROPRIATE BOX IF A MEMBER OF A GROUP*

(a)
¨

(b)
¨

3.

SEC USE ONLY

4.

SOURCE OF FUNDS*

OO

5.

CHECK
BOX IF DISCLOSURE OF LEGAL PROCEEDINGS IS REQUIRED PURSUANT TO ITEM 2(d) or 2(e)

¨

6.

CITIZENSHIP OR PLACE OF ORGANIZATION

Delaware

NUMBER
OF SHARES
BENEFICIALLY
OWNED
BY EACH
REPORTING
PERSON WITH
7.

SOLE VOTING POWER:

2,712,847 (1)

8.

SHARED VOTING POWER:

0

9.

SOLE DISPOSITIVE POWER:

2,712,847 (1)

10.

SHARED DISPOSITIVE POWER:

0

11.

AGGREGATE AMOUNT BENEFICIALLY OWNED BY EACH REPORTING PERSON:

2,712,847 (1)

12.

CHECK BOX IF THE AGGREGATE AMOUNT IN ROW (11) EXCLUDES CERTAIN SHARES*

¨

13.

PERCENT OF CLASS REPRESENTED BY AMOUNT IN ROW (11)

9.99% (2)

14.

TYPE OF REPORTING PERSON*

IA, PN

(1) Includes 163,386 shares of the common stock of Mirati
Pharmaceuticals, Inc. (the “Issuer”) underlying 163,386 shares of the Issuer’s common stock issuable based upon
exercise of the Pre-funded Warrants (as defined in Item 4 and subject to the limitations described in Item 5) directly held.
(2) Based on 26,993,621 shares of common stock outstanding
following the Offering (as defined in Item 5) as reported in the Issuer’s Prospectus filed with the Securities and Exchange
Commission (“SEC”) on November 16, 2017 and 163,386 shares of the Issuer’s common stock issuable upon the exercise
of the Pre-funded Warrants (as definted in Item 4).

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