United States

SECURITIES AND EXCHANGE COMMISSION

Washington, D.C. 20549

FORM 8-K

CURRENT REPORT

Pursuant to Section 13 or 15(d) of the

Securities Exchange Act of 1934

Date of Report (Date of earliest event reported): January 8, 2016

THERMO FISHER SCIENTIFIC INC.

(Exact name of registrant as specified in its charter)

Registrant’s Telephone Number, Including Area Code: (781) 622-1000

Not Applicable

(Former name or former address, if changed since last report)

Check the appropriate box below if the Form 8-K filing is intended to simultaneously satisfy the filing obligation of the registrant under any of the following provisions:

On January 8, 2016, Thermo Fisher Scientific Inc. (“Thermo Fisher”) and Affymetrix, Inc. (“Affymetrix”) issued a joint press release announcing the execution of an Agreement and Plan of Merger, pursuant to which, among other things and subject to the satisfaction or waiver of specified conditions, Thermo Fisher agreed to acquire Affymetrix in an all cash transaction. A copy of the joint press release is attached as Exhibit 99.1 hereto and is incorporated herein by reference.

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Additional Information

In connection with the proposed merger, Affymetrix will file a proxy statement with the Securities and Exchange Commission (the “SEC”). STOCKHOLDERS ARE ADVISED TO READ THE PROXY STATEMENT WHEN IT BECOMES AVAILABLE BECAUSE IT WILL CONTAIN IMPORTANT INFORMATION. Stockholders may obtain a free copy of the proxy statement (when available) and any other relevant documents filed with the SEC from the SEC’s website at http://www.sec.gov. In addition, stockholders will be able to obtain, without charge, a copy of the proxy statement and other relevant documents (when available) at Affymetrix’ website at investor.Affymetrix.com or by contacting Affymetrix’ investor relations department via e-mail at investor@affymetrix.com.

Participants in the Solicitation

Affymetrix and its directors, executive officers and other members of its management and employees as well as Thermo Fisher and its directors and executive officers may be deemed to be participants in the solicitation of proxies from Affymetrix’ stockholders with respect to the merger. Information about Affymetrix’ directors and executive officers and their ownership of Affymetrix’ common stock is set forth in the proxy statement for Affymetrix’ 2015 Annual Meeting of Stockholders and Affymetrix’ Annual Report on Form 10-K for the fiscal year ended December 31, 2014. Information about Thermo Fisher’s directors and executive officers is set forth in the proxy statement for Thermo Fisher’s 2015 Annual Meeting of Stockholders. Stockholders may obtain additional information regarding the direct and indirect interests of the participants in the solicitation of proxies in connection with the merger, including the interests of Affymetrix’ directors and executive officers in the merger, which may be different than those of Affymetrix’ stockholders generally, by reading the proxy statement and other relevant documents regarding the merger, which will be filed with the SEC.

SIGNATURES

Pursuant to the requirements of the Securities Exchange Act of 1934, the registrant has duly caused this report to be signed on its behalf by the undersigned hereunto duly authorized.

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