Sanofi — Investor Presentation 2011

Feb 16, 2011

Filed by sanofi-aventis

Pursuant to Rule 425 under the Securities Act of 1933, as amended

Subject Company: Genzyme Corporation

Exchange Act Commission File No: 0-14680

The following investor presentation, entitled “Adding a New Global Platform to our Sustainable Growth Strategy” was made by sanofi-aventis and Genzyme Corporation (“Genzyme”) on February 16, 2011 to investors of the companies, in connection with the Agreement and Plan of Merger, dated as of February 16, 2011, among sanofi-aventis, GC Merger Corp., and Genzyme.

Adding a New Global Platform to our Sustainable Growth Strategy February 16, 2011

2 Forward Looking Statements This presentation contains forward-looking statements as defined in the Private Securities Litigation Reform Act of 1995, as amended. Forward-looking statements are statements that are not historical facts. These statements include projections and estimates and their underlying assumptions, statements regarding plans, objectives, intentions and expectations with respect to future financial results, events, operations, services, product development and potential, and statements regarding future performance. Forward-looking statements are generally identified by the words “expects,” “anticipates,” “believes,” “intends,” “estimates,” “plans” and similar expressions. Although sanofi-aventis’ management believes that the expectations reflected in such forward-looking statements are reasonable, investors are cautioned that forward-looking information and statements are subject to various risks and uncertainties, many of which are difficult to predict and generally beyond the control of sanofi-aventis, that could cause actual results and developments to differ materially from those expressed in, or implied or projected by, the forward-looking information and statements. These risks and uncertainties include among other things, the uncertainties inherent in research and development, future clinical data and analysis, including post marketing, decisions by regulatory authorities, such as the FDA or the EMA, regarding whether and when to approve any drug, device or biological application that may be filed for any such product candidates as well as their decisions regarding labeling and other matters that could affect the availability or commercial potential of such products candidates, the absence of guarantee that the products candidates if approved will be commercially successful, the future approval and commercial success of therapeutic alternatives, the Group’s ability to benefit from external growth opportunities as well as those discussed or identified in the public filings with the SEC and the AMF made by sanofi-aventis, including those listed under “Risk Factors” and “Cautionary Statement Regarding Forward-Looking Statements” in sanofi-aventis’ annual report on Form 20-F for the year ended December 31, 2009. Other than as required by applicable law, sanofi-aventis does not undertake any obligation to update or revise any forward-looking information or statements. Important Information about this Transaction: This communication is neither an offer to purchase nor a solicitation of any offer to sell any securities. In connection with the proposed transaction, sanofi-aventis will file an amended tender offer statement and a registration statement on Form F-4 to register certain securities and certain related documents and Genzyme will file a Solicitation/Recommendation Statement with respect to the exchange offer with the U.S. Securities and Exchange Commission (the “SEC”). Genzyme shareholders are urged to read the registration statement and exchange offer documents when they become available because they will contain important information that shareholders should consider before making any decision regarding tendering their shares. These documents will be mailed to all Genzyme shareholders of record. These documents, as they may be amended from time to time, contain important information about the proposed transaction and Genzyme shareholders are urged to read them carefully and in their entirety before any decision is made with respect to the proposed transaction. When available, documentation relating to the transaction may be obtained at no charge at the website maintained by the SEC at www.sec.gov and may also be obtained at no charge by directing a request by mail to MacKenzie Partners, Inc., 105 Madison Avenue, New York, New York 10016, or by calling toll-free at (800) 322-2885. Free copies of the Solicitation/Recommendation Statement will be made available by Genzyme by directing a request to Genzyme at 500 Kendall Street, Cambridge, MA 02142, Attention: Shareholder Relations Department, or by calling 617-252-7500 and asking for the Shareholder Relations Department. This communication shall not constitute an offer to sell or the solicitation of an offer to buy any securities, nor shall there be any sale of securities in any jurisdiction in which such offer, solicitation or sale would be unlawful prior to registration or qualification under the securities laws of any such jurisdiction. No offering of securities shall be made except by means of a prospectus meeting the requirements of the U.S. Securities Act of 1933, as amended, or an exemption therefrom.

3 Conference Call Speakers sanofi-aventis Christopher A. Viehbacher Chief Executive Officer Jérôme Contamine Executive Vice President, Chief Financial Officer Genzyme Henri A. Termeer Chairman of the Board, President, Chief Executive Officer

Rationale for sanofi-aventis Christopher A. Viehbacher Chief Executive Officer

5 A Strategically Consistent and Financially Attractive Acquisition for sanofi-aventis Creates a meaningful new growth platform Expands our footprint in biotechnology Increases our U.S. presence Bolts on to our existing business Significantly contributes to our goal of sustainable earnings growth Drives significant long-term value for our shareholders

Rationale for Genzyme Henri A. Termeer Chairman of the Board, President, CEO

7 The Right Transaction for Genzyme, its Shareholders, and Other Stakeholders Represents a new beginning for Genzyme Tremendous value recognized by sanofi-aventis in our record of innovation in our unique and pioneering approach to serving patients Shared exciting vision of the future in which Genzyme and sanofi- aventis will grow and innovate Genzyme to help sanofi-aventis to grow and to innovate Sanofi-aventis believes in what we what we do, our people and our potential Looking forward to building a sustainable future together

Transaction Overview Jérôme Contamine Executive Vice President, Chief Financial Officer

9 Sanofi-aventis to Acquire Genzyme $74.00 per share in cash plus contingent value right (CVR) Risk / reward on Lemtrada™ and sustained production of Cerezyme® and Fabrazyme® shared through CVR structure CVR security to be publicly traded Transaction unanimously approved by both Boards of Directors No financing conditions Anti-trust clearance already received in the U.S. and EU Anticipated closing in early Q2 2011

10 CVR Structure (1) Production Milestone Approval Milestone Sales Milestone #1 Sales Milestone #2 Sales Milestone #3 Sales Milestone #4 Cash Payments $1.00 per CVR $1.00 per CVR $2.00 per CVR $3.00 per CVR $4.00 per CVR $3.00 per CVR Triggers Paid if specified Cerezyme® / Fabrazyme® production levels are met in 2011 Paid upon final FDA approval of Lemtrada™ for multiple sclerosis (MS) indication Paid if Lemtrada™ net sales post launch exceed an aggregate of $400m within four specified quarters per territory Paid if and when Lemtrada™ global net sales exceed $1,800m Paid if and when Lemtrada™ global net sales exceed $2,300m Paid if and when Lemtrada™ global net sales exceed $2,800m Estimated Timing of Potential Payments Jan 2012 ~H2 2012 ~2014 2014-2020 2015-2020 2016-2020 Upside potential if Lemtrada™ is approved for Multiple Sclerosis to be shared between Genzyme and sanofi-aventis shareholders at all levels of sales performance The CVR will be publicly traded on NASDAQ (1) Details are defined in the Merger Agreement and CVR Agreement to be filed with the SEC

11 Genzyme Acquisition Delivers Significant Financial Benefits Revenue growth acceleration Genzyme to benefit from sanofi-aventis’ extensive resources and experience in manufacturing Significant Business EPS accretion expected: Accretive in Year 1 after closing Accretion of € 0.75 to € 1.00 by 2013 (1) Value-creating transaction: Return on capital in excess of WACC expected by Year 2 Financial strength and flexibility maintained: No change to sanofi-aventis dividend policy Strong free cash flow generated by combination Strong investment grade credit ratings maintained (1) At exchange rate of €1 = $1.30

12 Transaction valued at $20.1bn , excluding CVR component Net cash position of Genzyme: ~$800m by end of 2010 Fully financed upfront cash transaction Bridge facilities ( $10bn to be refinanced by bond issuance and $5bn to be refinanced by cash flow) Cash and/or issuance of commercial paper Attractive cost of debt: 2.5% to 3% Additional existing syndicated revolving credit facilities ( € 13bn ) Sanofi-aventis Has Already Secured Acquisition Financing

Next Steps Christopher A. Viehbacher Chief Executive Officer

14 A Clear Path Forward given Antitrust Clearance already Granted in the U.S. and EU Q1 Q2 Q3 Legal process Tender offer, CVR registration Feb 16 Merger announcement Early Q2 Anticipated closing Planning Integration Mid Feb Kick-off Legal process Pre-integration process Integration process Day 1 2011

Q&A