13G Filing: Viking Global and aTyr Pharma Inc (LIFE)

Schedule 13G PAGE 14 of 14

CUSIP No. 002120103

EXHIBIT A – JOINT FILING AGREEMENT

This joint filing agreement is made and entered into as of
this 8th day of September, 2017, by and among Viking Global Investors LP,
Viking Global Opportunities GP LLC, Viking Global Opportunities
Portfolio GP LLC, Viking Global Opportunities Illiquid Investments
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 aTyr Pharma, Inc., 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: September 8, 2017

/s/ O. ANDREAS HALVORSEN
By: O. Andreas Halvorsen – individually 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 GLOBAL OPPORTUNITIES GP LLC, on behalf
of itself and VIKING GLOBAL OPPORTUNITIES PORTFOLIO GP LLC and
VIKING GLOBAL OPPORTUNITIES ILLIQUID INVESTMENTS SUB-MASTER LP

/s/ DAVID C. OTT
By: David C. Ott – individually 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 GLOBAL OPPORTUNITIES GP LLC, on behalf
of itself and VIKING GLOBAL OPPORTUNITIES PORTFOLIO GP LLC and
VIKING GLOBAL OPPORTUNITIES ILLIQUID INVESTMENTS SUB-MASTER LP

/s/ ROSE S. SHABET
By: Rose S. Shabet – individually 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 GLOBAL OPPORTUNITIES GP LLC, on behalf
of itself and VIKING GLOBAL OPPORTUNITIES PORTFOLIO GP LLC and
VIKING GLOBAL OPPORTUNITIES ILLIQUID INVESTMENTS SUB-MASTER LP