Cubist Pharmaceuticals, Inc. (NASDAQ: CBST) and Optimer Pharmaceuticals, Inc. (NASDAQ: OPTR) today announced the expiration of the waiting period under the Hart-Scott-Rodino Antitrust Improvements Act of 1976 (“HSR”) in connection with the previously announced merger agreement under which Cubist will acquire all of the outstanding common stock of Optimer. Expiration of the HSR waiting period satisfies one of the conditions to the closing of the merger. The proposed merger remains subject to approval by the stockholders of Optimer, as well as other customary closing conditions. Upon closing of the merger, each holder of Optimer common stock will receive $10.75 per share in cash, plus one Contingent Value Right, entitling the holder to receive an additional one-time cash payment of up to $5.00 for each share they own if certain net sales of DIFICID® (fidaxomicin) in the United States and Canada are achieved. About Optimer Pharmaceuticals Optimer is a global biopharmaceutical company currently focused on commercializing its antibiotic product DIFICID® (fidaxomicin) tablets in the United States and Canada, and developing other fidaxomicin products in the United States and worldwide, both independently and with its partners and licensees. DIFICID, a macrolide antibacterial drug, was approved by the U.S. Food and Drug Administration on May 27, 2011, for the treatment of Clostridium difficile-associated diarrhea, or CDAD, in adults 18 years of age and older. Fidaxomicin also has received marketing authorization in other jurisdictions, including the European Union, where it is marketed under the trade name DIFICLIR™ by Optimer’s licensee, Astellas Pharma Europe. CDAD is the most common symptom of Clostridium difficile infection, or CDI. Additional information can be found at http://www.optimerpharma.com. About Cubist Cubist Pharmaceuticals, Inc. is a biopharmaceutical company focused on the research, development, and commercialization of pharmaceutical products that address significant unmet medical needs in the acute care environment. Cubist is headquartered in Lexington, Mass. Additional information can be found at Cubist’s web site at www.cubist.com. Cautionary Note Regarding Forward-Looking Statements This press release contains forward-looking statements within the meaning of the Private Securities Litigation Reform Act of 1995 and other federal securities laws. Any statements contained herein which do not describe historical facts, including but not limited to, statements regarding the proposed transaction between Cubist and Optimer, are forward-looking statements which involve risks and uncertainties that could cause actual results to differ materially from those discussed in such forward-looking statements. Such risks and uncertainties include: the possibility that certain conditions to the completion of the transaction between Optimer and Cubist are not satisfied, or that the transaction may otherwise not be completed in a timely manner, or at all; that, prior to the completion of the transaction, if at all, Optimer may not satisfy one or more closing conditions; that the merger agreement may be terminated; the impact of the proposed transaction on the business, employees, customers, suppliers and commercial partners of Cubist and Optimer; the ability of Cubist to successfully integrate Optimer’s operations and employees; the ability of Cubist to achieve the net sales required to trigger a future cash payment in respect of the contingent value right component of the merger consideration; the anticipated benefits of the transaction may not be realized; risks related to drug development and commercialization; and those additional factors discussed in Cubist’s and Optimer’s most recent Quarterly and Annual Reports on Forms 10-Q and 10-K filed with the Securities and Exchange Commission (“SEC”). Cubist and Optimer caution investors not to place considerable reliance on the forward-looking statements contained in this press release. These forward-looking statements speak only as of the date of this document, and Cubist and Optimer undertake no obligation to update or revise any of these statements.