Ownership Submission
FORM 4
Check this box if no longer subject to Section 16. Form 4 or Form 5 obligations may continue. See Instruction 1(b).
UNITED STATES SECURITIES AND EXCHANGE COMMISSION
Washington, D.C. 20549

STATEMENT OF CHANGES IN BENEFICIAL OWNERSHIP OF SECURITIES

Filed pursuant to Section 16(a) of the Securities Exchange Act of 1934, Section 17(a) of the Public Utility Holding Company Act of 1935 or Section 30(h) of the Investment Company Act of 1940
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(Print or Type Responses)
1. Name and Address of Reporting Person *
ATLAS VENTURE FUND VI LP
  2. Issuer Name and Ticker or Trading Symbol
HORIZON PHARMA, INC. [HZNP]
5. Relationship of Reporting Person(s) to Issuer
(Check all applicable)
_____ Director __X__ 10% Owner
_____ Officer (give title below) _____ Other (specify below)
(Last)
(First)
(Middle)
C/O ATLAS VENTURE, 25 FIRST STREET, SUITE 303
3. Date of Earliest Transaction (Month/Day/Year)
08/02/2011
(Street)

CAMBRIDGE, MA 02141
4. If Amendment, Date Original Filed(Month/Day/Year)
6. Individual or Joint/Group Filing(Check Applicable Line)
___ Form filed by One Reporting Person
_X_ Form filed by More than One Reporting Person
(City)
(State)
(Zip)
Table I - Non-Derivative Securities Acquired, Disposed of, or Beneficially Owned
1.Title of Security
(Instr. 3)
2. Transaction Date (Month/Day/Year) 2A. Deemed Execution Date, if any (Month/Day/Year) 3. Transaction Code
(Instr. 8)
4. Securities Acquired (A) or Disposed of (D)
(Instr. 3, 4 and 5)
5. Amount of Securities Beneficially Owned Following Reported Transaction(s)
(Instr. 3 and 4)
6. Ownership Form: Direct (D) or Indirect (I)
(Instr. 4)
7. Nature of Indirect Beneficial Ownership
(Instr. 4)
Code V Amount (A) or (D) Price
Common Stock 08/02/2011   C   1,577,819 (1) (2) A (1) 1,577,819 I See FN (3)
Common Stock 08/02/2011   C   179,740 (1) (4) A (1) 1,757,559 I See FN (3)
Common Stock 08/02/2011   C   387,034 (5) (6) A $ 9 2,144,593 I See FN (3)
Common Stock 08/02/2011   C   7,086 (5) (7) A $ 9 2,151,679 I See FN (3)
Common Stock 08/02/2011   C   11,834 (5) (8) A $ 9 2,163,513 I See FN (3)
Common Stock 08/02/2011   P   352,576 (6) A $ 9 2,516,089 I See FN (3)
Common Stock 08/02/2011   P   6,456 (7) A $ 9 2,522,545 I See FN (3)
Common Stock 08/02/2011   P   10,782 (8) A $ 9 2,533,327 I See FN (3)
Common Stock               2,859,851 I See FN (9)

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Table II - Derivative Securities Acquired, Disposed of, or Beneficially Owned
(e.g., puts, calls, warrants, options, convertible securities)
1. Title of Derivative Security
(Instr. 3)
2. Conversion or Exercise Price of Derivative Security 3. Transaction Date (Month/Day/Year) 3A. Deemed Execution Date, if any (Month/Day/Year) 4. Transaction Code
(Instr. 8)
5. Number of Derivative Securities Acquired (A) or Disposed of (D)
(Instr. 3, 4, and 5)
6. Date Exercisable and Expiration Date
(Month/Day/Year)
7. Title and Amount of Underlying Securities
(Instr. 3 and 4)
8. Price of Derivative Security
(Instr. 5)
9. Number of Derivative Securities Beneficially Owned Following Reported Transaction(s)
(Instr. 4)
10. Ownership Form of Derivative Security: Direct (D) or Indirect (I)
(Instr. 4)
11. Nature of Indirect Beneficial Ownership
(Instr. 4)
Code V (A) (D) Date Exercisable Expiration Date Title Amount or Number of Shares
Series A Convertible Preferred Stock (1) 08/02/2011   C     1,577,819   (10)   (10) Common Stock 1,577,819 $ 0 0 I See FN (2)
Series B Convertible Preferred Stock (1) 08/02/2011   C     179,740   (10)   (10) Common Stock 179,740 $ 0 0 I See FN (4)
Convertible Promissory Notes $ 9 08/02/2011   C     $ 3,483,320.16   (11)   (11) Common Stock 387,034 $ 0 0 I See FN (6)
Convertible Promissory Notes $ 9 08/02/2011   C     $ 63,784.09   (11)   (11) Common Stock 7,086 $ 0 0 I See FN (7)
Convertible Promissory Notes $ 9 08/02/2011   C     $ 106,520.53   (11)   (11) Common Stock 11,834 $ 0 0 I See FN (8)

Reporting Owners

Reporting Owner Name / Address Relationships
 Director  10% Owner  Officer  Other
ATLAS VENTURE FUND VI LP
C/O ATLAS VENTURE
25 FIRST STREET, SUITE 303
CAMBRIDGE, MA 02141
    X    
Atlas Venture Associates VI, Inc
C/O ATLAS VENTURE
25 FIRST STREET, SUITE 303
CAMBRIDGE, MA 02141
    X    
ATLAS VENTURE ENTREPRENEURS FUND VI LP
C/O ATLAS VENTURE
25 FIRST STREET, SUITE 303
CAMBRIDGE, MA 02141
    X    
Atlas Venture Fund VI GmbH & Co KG
C/O ATLAS VENTURE
25 FIRST STREET, SUITE 303
CAMBRIDGE, MA 02141
    X    

Signatures

 /s/ Kristen Laguerre, Vice President of Atlas Venture Associates VI, Inc., the general partner of Atlas Venture Associates VI, L.P., the general partner of Atlas Venture Fund VI, L.P.   08/04/2011
**Signature of Reporting Person Date

 /s/ Kristen Laguerre, Vice President of Atlas Venture Associates VI, Inc., the general partner of Atlas Venture Associates VI, Inc.   08/04/2011
**Signature of Reporting Person Date

 /s/ Kristen Laguerre, Vice President of Atlas Venture Associates VI, Inc., the general partner of Atlas Venture Associates VI, L.P., the general partner of Atlas Venture Entrepreneurs' Fund VI, L.P.   08/04/2011
**Signature of Reporting Person Date

 /s/ Kristen Laguerre, Vice President of Atlas Venture Associates VI, Inc., the general partner of Atlas Venture Associates VI, L.P., the managing limited partner of Atlas Venture Fund VI GmbH & Co. KG   08/04/2011
**Signature of Reporting Person Date

Explanation of Responses:

* If the form is filed by more than one reporting person, see Instruction 4(b)(v).
** Intentional misstatements or omissions of facts constitute Federal Criminal Violations. See 18 U.S.C. 1001 and 15 U.S.C. 78ff(a).
(1) Automatic conversion, in connection with the Issuer's initial public offering, of preferred stock into shares of common stock. Every 2.374 shares of preferred stock converted into one share of common stock upon the closing of the Issuer's initial public offering.
(2) Includes 46,003 shares held of record by Atlas Venture Entrepreneurs' Fund VI, L.P. ("AVE"), 27,544 shares held of record by Atlas Fund VI GmbH & Co. KG ("GmbH") and 1,504,272 shares held by Atlas Venture Fund VI, L.P. ("Venture"). These shares are owned directly by AVE, GmbH and Venture. Atlas Venture Associates VI, L.P. disclaims beneficial ownership as to such shares except to the extent of its proportionate pecuniary interest therein and each of AVE, GmbH and Venture disclaim beneficial ownership to such shares other than those shares held directly by them.
(3) These shares are owned directly by AVE, GmbH and Venture. Atlas Venture Associates VI, L.P. disclaims beneficial ownership as to such shares except to the extent of its proportionate pecuniary interest therein and each of AVE, GmbH and Venture disclaim beneficial ownership to such shares other than those shares held directly by them.
(4) Includes 5,240 shares held of record by AVE, 3,137 shares held of record by GmbH and 171,363 shares held by Venture. These shares are owned directly by AVE, GmbH and Venture. Atlas Venture Associates VI, L.P. disclaims beneficial ownership as to such shares except to the extent of its proportionate pecuniary interest therein and each of AVE, GmbH and Venture disclaim beneficial ownership to such shares other than those shares held directly by them.
(5) Represents shares acquired upon conversion, in connection with the Issuer's initial public offering, of convertible promissory notes.
(6) Securities held in the name of Venture. These shares are owned directly by AVE, GmbH and Venture. Atlas Venture Associates VI, L.P. disclaims beneficial ownership as to such shares except to the extent of its proportionate pecuniary interest therein and each of AVE, GmbH and Venture disclaim beneficial ownership to such shares other than those shares held directly by them.
(7) Securities held in the name of GmbH. These shares are owned directly by AVE, GmbH and Venture. Atlas Venture Associates VI, L.P. disclaims beneficial ownership as to such shares except to the extent of its proportionate pecuniary interest therein and each of AVE, GmbH and Venture disclaim beneficial ownership to such shares other than those shares held directly by them.
(8) Securities held in the name of AVE. These shares are owned directly by AVE, GmbH and Venture. Atlas Venture Associates VI, L.P. disclaims beneficial ownership as to such shares except to the extent of its proportionate pecuniary interest therein and each of AVE, GmbH and Venture disclaim beneficial ownership to such shares other than those shares held directly by them.
(9) Includes 83,379 shares held of record by AVE, 49,923 shares held of record by GmbH and 2,726,549 shares held by Venture. These shares are owned directly by AVE, GmbH and Venture. Atlas Venture Associates VI, L.P. disclaims beneficial ownership as to such shares except to the extent of its proportionate pecuniary interest therein and each of AVE, GmbH and Venture disclaim beneficial ownership to such shares other than those shares held directly by them.
(10) These securities are preferred stock of the Issuer and did not have an expiration date. Prior to the Issuer's initial public offering of common stock, the securities were convertible at any time at the election of the holder. The securities automatically converted into shares of common stock upon the closing of the issuer's initial public offering of common stock.
(11) Principal and accrued interest converted upon the Issuer's initial public offering of common stock, into shares of common stock.

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