UNITED STATES

SECURITIES AND EXCHANGE COMMISSION

Washington, D.C. 20549

o

Rule 13d-1(b)

o

Rule 13d-1(c)

x

Rule 13d-1(d)

CUSIP No. 00211Y100

13G

1.

Name of Reporting Person
S.S. or I.R.S. Identification No. of Above Person

Atlas Venture Fund VII, L.P.

2.

Check the Appropriate Box if a Member of a Group*

(a)

o

(b)

o

3.

SEC Use Only

4.

Citizenship or Place of Organization
Delaware

Number of
Shares
Beneficially
Owned by
Each
Reporting
Person With

5.

Sole Voting Power

6.

Shared Voting Power
1,893,025 shares

7.

Sole Dispositive Power

8.

Shared Dispositive Power
1,893,025 shares

9.

Aggregate Amount Beneficially Owned by Each Reporting Person
1,893,025 shares

10.

Check Box if the Aggregate Amount in Row (9) Excludes Certain Shares  o

11.

Percent of Class Represented by Amount in Row (9)
12.26%

12.

Type of Reporting Person*
PN

Item 1(a).

Name of Issuer
The name of the issuer to which this filing on Schedule 13G/A relates is ARCA biopharma, Inc. (the “Company”).

Item 1(b).

Address of Issuer’s Principal Executive Offices
The principal executive offices of the Company are located at 8001 Arista Place, Suite 200, Broomfield, CO 80021.

Item 2(a).

Name of Person Filing
This Statement is being filed on behalf of Atlas Venture Fund VII, L.P. (“Atlas VII”). Atlas Venture Associates VII, L.P. (“AVA VII LP”) is the sole general partner of Atlas VII.  Atlas Venture Associates VII, Inc. (“AVA VII Inc.”) is the sole general partner of AVA VII LP.  Jean-Francois Formela, Peter Barrett and Jeff Fagnan is each a director of AVA VII Inc. (the “Directors”).

Item 2(b).

Address of Principal Business Office or, if none, Residence
The principal business address of each of Atlas VII, AVA VII LP, AVA VII Inc. and the Directors is 25 First Street, Suite 303, Cambridge MA 02141.

Item 2(c).

Citizenship
Each of Atlas VII, AVA VII LP and AVA VII Inc. is organized under the laws of Delaware.  Mr. Formela is a citizen of France.  Mr. Barrett is a citizen of the United States.  Mr. Fagnan is a citizen of the United States.

Item 2(d).

Title of Class of Securities
The class of equity securities of the Company to which this filing on Schedule 13G/A relates is Common Stock, par value $0.001 (“Common Stock”).

Item 2(e).

CUSIP Number
The CUSIP number of the Company’s Common Stock is 00211Y100.

Item 3.

If this statement is filed pursuant to §§240.13d-1(b) or 240.13d-2(b) or (c), check whether the person filing is a:    Not applicable.

(a)

o

Broker or dealer registered under section 15 of the Act (15 U.S.C. 78o).

(b)

o

Bank as defined in section 3(a)(6) of the Act (15 U.S.C. 73c).

(c)

o

Insurance company as defined in section 3(a)(19) of the Act (15 U.S.C. 78c).

(d)

o

Investment company registered under section 8 of the Investment Company Act of 1940 (15 U.S.C. 80a-8).

(e)

o

An investment adviser in accordance with §13d-1(b)(1)(ii)(E).

(f)

o

An employee benefit plan or endowment fund in accordance with §240.13d-1(b)(1)(ii)(F).

(g)

o

A parent holding company or control person in accordance with §240.13d-1(b)(1)(ii)(G).

(h)

o

A savings association as defined in Section 3(b) of the Federal Deposit Insurance Act (12 U.S.C. 1813).

(i)

o

A church plan that is excluded from the definition of an investment company under section 3(c)(14) of the Investment Company Act of 1940 (15 U.S.C. 80a-3).

(j)

o

Group, in accordance with §240.13d-1(b)(1)(ii)(J).

Item 4.

Ownership

Item 4(a).

Amount beneficially owned
As of the close of business on December 31, 2012, Atlas VII is the record holder of 1,579,542 shares of Common Stock and 313,483 warrants to purchase shares of Common Stock (the “Atlas Shares”).  AVA VII LP is the sole general partner of Atlas VII.  AVA VII Inc. is the sole general partner of AVA VII LP.  Messrs. Formela, Barrett and Fagnan are the directors of AVA VII Inc.  No person other than the respective owner referred to herein of the Atlas Shares is known to have the right to receive or the power to direct the receipt of dividends from or the proceeds from the sale of such Atlas Shares.  Each of Atlas VII, AVA VII LP, AVA VII Inc. and the Directors disclaim beneficial ownership of the Atlas Shares except for such shares, if any, such person holds of record.

Item 4(b).

Percent of class
As of the close of business on December 31, 2012 Atlas VII was the beneficial owner of 12.26% of the Common Stock.

Item 4(c).

Number of shares as to which the person has:

(i)

Sole power to vote or to direct the vote:   

0

(ii)

Shared power to vote or direct the vote:    

1,893,025 shares

(iii)

Sole power to dispose or direct the disposition of:   

0

(iv)

Shared power to dispose or direct the disposition of:   

1,893,025 shares

Item 5.

Ownership of Five Percent or Less of a Class

Not Applicable.

Item 6.

Ownership of More than Five Percent on Behalf of Another Person

Not Applicable.

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.  The Filing Persons expressly disclaim membership in a “group” as used in Rule 13d-5(b)(1).

Item 9.

Notice of Dissolution of Group

Not Applicable.

Item 10.

Certification

Not Applicable.  This statement on Schedule 13G is not filed pursuant to Rule 13d-1(b) or 13(d)-1(c).

ATLAS VENTURE FUND VII, L.P.

By:

Atlas Venture Associates VII, L.P., its general partner

By:

Atlas Venture Associates VII, Inc., its general partner

By:

/s/ Kristen Laguerre

Name:

Kristen Laguerre

Title:

Vice President