The Sirtris board of directors has unanimously determined that the merger agreement, the tender offer and the merger are advisable and are fair to, and in the best interests of, Sirtris and Sirtris' stockholders and unanimously recommends that Sirtris' stockholders tender their shares in the offer.

The tender offer will expire at 12:00 midnight on Friday, May 30, 2008, unless extended in accordance with the merger agreement and the applicable rules and regulations of the Securities and Exchange Commission (SEC). The offer is subject to customary conditions, including the acquisition by GSK of a majority of the outstanding shares of Sirtris common stock on a fully diluted basis.

About GlaxoSmithKline plc
GlaxoSmithKline plc - one of the world's leading research-based pharmaceutical and healthcare companies - is committed to improving the quality of human life by enabling people to do more, feel better and live longer. For company information including a copy of this announcement and details of the company's updated product development pipeline, visit GSK at www.gsk.com.

About Sirtris
Sirtris Pharmaceuticals, Inc. is a biopharmaceutical company focused on discovering and developing proprietary, orally available, small molecule drugs with the potential to treat diseases associated with ageing, including metabolic diseases such as Type 2 Diabetes. Sirtris's drug candidates are designed to mimic certain beneficial health effects of calorie restriction by activation of sirtuins, a recently discovered class of enzymes that Sirtris believes control the ageing process. The company's headquarters are in Cambridge, Massachusetts.