Filing Details

SECURITIES AND EXCHANGE COMMISSION Washington, D.C. 20549 ________________ Schedule 13G/A ________________ 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* (Amendment No. 1) aTyr Pharma, Inc. (Name of Issuer) Common stock, par value $0.001 per share (Title of Class of Securities) 002120103 (CUSIP Number) December 31, 2017 (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/A PAGE 2 of 14 CUSIP No. 002120103 _____________________________________________________________________________ (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 2,978,490* OWNED BY ______________________________________________________________ EACH (7) SOLE DISPOSITIVE POWER 0 REPORTING ______________________________________________________________ PERSON WITH (8) SHARED DISPOSITIVE POWER 2,978,490* _____________________________________________________________________________ (9) AGGREGATE AMOUNT BENEFICIALLY OWNED BY EACH REPORTING PERSON 2,978,490* _____________________________________________________________________________ (10) CHECK BOX IF THE AGGREGATE AMOUNT IN ROW (9) EXCLUDES CERTAIN SHARES [x] _____________________________________________________________________________ (11) PERCENT OF CLASS REPRESENTED BY AMOUNT IN ROW (9) 9.5%* _____________________________________________________________________________ (12) TYPE OF REPORTING PERSON PN _____________________________________________________________________________ *See Item 4 Schedule 13G/A PAGE 3 of 14 CUSIP No. 002120103 _____________________________________________________________________________ (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 2,978,490* OWNED BY ______________________________________________________________ EACH (7) SOLE DISPOSITIVE POWER 0 REPORTING ______________________________________________________________ PERSON WITH (8) SHARED DISPOSITIVE POWER 2,978,490* _____________________________________________________________________________ (9) AGGREGATE AMOUNT BENEFICIALLY OWNED BY EACH REPORTING PERSON 2,978,490* _____________________________________________________________________________ (10) CHECK BOX IF THE AGGREGATE AMOUNT IN ROW (9) EXCLUDES CERTAIN SHARES [x] _____________________________________________________________________________ (11) PERCENT OF CLASS REPRESENTED BY AMOUNT IN ROW (9) 9.5%* _____________________________________________________________________________ (12) TYPE OF REPORTING PERSON OO _____________________________________________________________________________ *See Item 4 Schedule 13G/A PAGE 4 of 14 CUSIP No. 002120103 _____________________________________________________________________________ (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 2,978,490* OWNED BY ______________________________________________________________ EACH (7) SOLE DISPOSITIVE POWER 0 REPORTING ______________________________________________________________ PERSON WITH (8) SHARED DISPOSITIVE POWER 2,978,490* _____________________________________________________________________________ (9) AGGREGATE AMOUNT BENEFICIALLY OWNED BY EACH REPORTING PERSON 2,978,490* _____________________________________________________________________________ (10) CHECK BOX IF THE AGGREGATE AMOUNT IN ROW (9) EXCLUDES CERTAIN SHARES [x] _____________________________________________________________________________ (11) PERCENT OF CLASS REPRESENTED BY AMOUNT IN ROW (9) 9.5%* _____________________________________________________________________________ (12) TYPE OF REPORTING PERSON OO _____________________________________________________________________________ *See Item 4 Schedule 13G/A PAGE 5 of 14 CUSIP No. 002120103 _____________________________________________________________________________ (1) NAME OF REPORTING PERSON S.S. OR I.R.S. IDENTIFICATION NO. OF ABOVE PERSON Viking Global Opportunities Illiquid Investments 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 2,978,490* OWNED BY ______________________________________________________________ EACH (7) SOLE DISPOSITIVE POWER 0 REPORTING ______________________________________________________________ PERSON WITH (8) SHARED DISPOSITIVE POWER 2,978,490* _____________________________________________________________________________ (9) AGGREGATE AMOUNT BENEFICIALLY OWNED BY EACH REPORTING PERSON 2,978,490* _____________________________________________________________________________ (10) CHECK BOX IF THE AGGREGATE AMOUNT IN ROW (9) EXCLUDES CERTAIN SHARES [x] _____________________________________________________________________________ (11) PERCENT OF CLASS REPRESENTED BY AMOUNT IN ROW (9) 9.5%* _____________________________________________________________________________ (12) TYPE OF REPORTING PERSON PN _____________________________________________________________________________ *See Item 4 Schedule 13G/A PAGE 6 of 14 CUSIP No. 002120103 _____________________________________________________________________________ (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 2,978,490* OWNED BY ______________________________________________________________ EACH (7) SOLE DISPOSITIVE POWER 0 REPORTING ______________________________________________________________ PERSON WITH (8) SHARED DISPOSITIVE POWER 2,978,490* _____________________________________________________________________________ (9) AGGREGATE AMOUNT BENEFICIALLY OWNED BY EACH REPORTING PERSON 2,978,490* _____________________________________________________________________________ (10) CHECK BOX IF THE AGGREGATE AMOUNT IN ROW (9) EXCLUDES CERTAIN SHARES [x] _____________________________________________________________________________ (11) PERCENT OF CLASS REPRESENTED BY AMOUNT IN ROW (9) 9.5%* _____________________________________________________________________________ (12) TYPE OF REPORTING PERSON IN _____________________________________________________________________________ *See Item 4 Schedule 13G/A PAGE 7 of 14 CUSIP No. 002120103 _____________________________________________________________________________ (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 2,978,490* OWNED BY ______________________________________________________________ EACH (7) SOLE DISPOSITIVE POWER 0 REPORTING ______________________________________________________________ PERSON WITH (8) SHARED DISPOSITIVE POWER 2,978,490* _____________________________________________________________________________ (9) AGGREGATE AMOUNT BENEFICIALLY OWNED BY EACH REPORTING PERSON 2,978,490* _____________________________________________________________________________ (10) CHECK BOX IF THE AGGREGATE AMOUNT IN ROW (9) EXCLUDES CERTAIN SHARES [x] _____________________________________________________________________________ (11) PERCENT OF CLASS REPRESENTED BY AMOUNT IN ROW (9) 9.5%* _____________________________________________________________________________ (12) TYPE OF REPORTING PERSON IN _____________________________________________________________________________ *See Item 4 Schedule 13G/A PAGE 8 of 14 CUSIP No. 002120103 _____________________________________________________________________________ (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 2,978,490* OWNED BY ______________________________________________________________ EACH (7) SOLE DISPOSITIVE POWER 0 REPORTING ______________________________________________________________ PERSON WITH (8) SHARED DISPOSITIVE POWER 2,978,490* _____________________________________________________________________________ (9) AGGREGATE AMOUNT BENEFICIALLY OWNED BY EACH REPORTING PERSON 2,978,490* _____________________________________________________________________________ (10) CHECK BOX IF THE AGGREGATE AMOUNT IN ROW (9) EXCLUDES CERTAIN SHARES [x] _____________________________________________________________________________ (11) PERCENT OF CLASS REPRESENTED BY AMOUNT IN ROW (9) 9.5%* _____________________________________________________________________________ (12) TYPE OF REPORTING PERSON IN _____________________________________________________________________________ *See Item 4 Schedule 13G/A PAGE 9 of 14 CUSIP No. 002120103 ITEM 1(a). NAME OF ISSUER: aTyr Pharma, Inc. ITEM 1(b). ADDRESS OF ISSUER'S PRINCIPAL EXECUTIVE OFFICES: 3545 John Hopkins Court, Suite #250 San Diego, California 92121 ITEM 2(a). NAME OF PERSON FILING: Viking Global Investors LP ("VGI"), Viking Global Opportunities GP LLC ("Opportunities GP"), Viking Global Opportunities Portfolio GP LLC ("Opportunities Portfolio GP"), Viking Global Opportunities Illiquid Investments 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 is a Delaware limited partnership; 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 $0.001 per share ("Common Stock") ITEM 2(e). CUSIP NUMBER: 002120103 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/A PAGE 10 of 14 CUSIP No. 002120103 (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: 2,978,490 (b) Percent of class: 9.5% (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 2,978,490 (iii) sole power to dispose or to direct the disposition of 0 (iv) shared power to dispose or to direct the disposition of 2,978,490 VGI provides managerial services to 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 Opportunities Fund. VGI does not directly own any shares of Common Stock. VGI beneficially owns 2,978,490 shares of Common Stock consisting of (i) 1,384,300 shares of Common Stock directly and beneficially owned by Opportunities Fund and (ii) 1,594,190 shares of Common Stock underlying Class X Preferred Stock, par value $0.001 per share (the "Preferred Stock") directly and beneficially owned by Opportunities Fund. Schedule 13G/A PAGE 11 of 14 CUSIP No. 002120103 B. Opportunities GP (a) Amount beneficially owned: 2,978,490 (b) Percent of class: 9.5% (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 2,978,490 (iii) sole power to dispose or to direct the disposition of 0 (iv) shared power to dispose or to direct the disposition of 2,978,490 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. Opportunities GP beneficially owns 2,978,490 shares of Common Stock consisting of (i) 1,384,300 shares of Common Stock directly and beneficially owned by Opportunities Fund and (ii) 1,594,190 shares of Common Stock underlying the Preferred Stock directly and beneficially owned by Opportunities Fund. C. Opportunities Portfolio GP (a) Amount beneficially owned: 2,978,490 (b) Percent of class: 9.5% (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 2,978,490 (iii) sole power to dispose or to direct the disposition of 0 (iv) shared power to dispose or to direct the disposition of 2,978,490 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. Opportunities Portfolio GP beneficially owns 2,978,490 shares of Common Stock consisting of (i) 1,384,300 shares of Common Stock directly and beneficially owned by Opportunities Fund and (ii) 1,594,190 shares of Common Stock underlying the Preferred Stock directly and beneficially owned by Opportunities Fund. D. Opportunities Fund (a) Amount beneficially owned: 2,978,490 (b) Percent of class: 9.5% (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 2,978,490 (iii) sole power to dispose or to direct the disposition of 0 (iv) shared power to dispose or to direct the disposition of 2,978,490 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. Opportunities Fund directly and beneficially owns 2,978,490 shares of Common Stock consisting of (i) 1,384,300 shares of Common Stock and (ii) 1,594,190 shares of Common Stock underlying the Preferred Stock. Schedule 13G/A PAGE 12 of 14 CUSIP No. 002120103 E. O. Andreas Halvorsen, David C. Ott and Rose S. Shabet (a) Amount beneficially owned: 2,978,490 (b) Percent of class: 9.5% (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 2,978,490 (iii) sole power to dispose or to direct the disposition of 0 (iv) shared power to dispose or to direct the disposition of 2,978,490 Mr. Halvorsen, Mr. Ott and Ms. Shabet, as Executive Committee Members of Viking Global Partners LLC ("VGPL"), general partner of VGI and Opportunities GP, have shared authority to dispose of and vote the shares of Common Stock beneficially owned by VGI 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 Opportunities Fund. Mr. Halvorsen, Mr. Ott and Ms. Shabet each beneficially own 2,978,490 shares of Common Stock consisting of (i) 1,384,300 shares of Common Stock directly and beneficially owned by Opportunities Fund and (ii) 1,594,190 shares of Common Stock underlying the Preferred Stock directly and beneficially owned by Opportunities Fund. Excluded from each Reporting Person's beneficial ownership are (i) 9,835,570 shares of Common Stock issuable upon the conversion of shares of Preferred Stock directly owned by Opportunities Fund and (ii) 4,952,829 shares of Common Stock issuable upon the exercise of warrants directly owned by Opportunities Fund due to a conversion cap that precludes Opportunities Fund from converting shares of Preferred Stock and exercising such warrants to the extent that Opportunities Fund would, after such conversion or exercise, beneficially own (as determined in accordance with Section 13(d) of the Act) in excess of 9.5% of the shares of Common Stock outstanding (the "Beneficial Ownership Limitation"). The percentage of the class of Common Stock beneficially owned by each Reporting Person is calculated based upon (i) 29,758,347 shares of Common Stock issued and outstanding as of November 7, 2017, as reported by the Issuer in its quarterly report on Form 10-Q filed with the Securities and Exchange Commission (the "Commission") on November 14, 2017 and (ii) the 1,594,190 shares of Common Stock underlying shares of Preferred Stock that could be converted by Opportunities Fund without violating the Beneficial Ownership Limitation. 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/A PAGE 13 of 14 CUSIP No. 002120103 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: February 14, 2018 /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 Schedule 13G/A 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