Gilead Sciences, the leading player in the HIV therapy market, has agreed to purchase Pharmasset for approximately US$11 B in order to advance its endeavours to develop an all-oral, interferon-free treatment regimen for hepatitis C virus (HCV) infection. Gilead will pay US$137 per share in cash, which represents an 89% premium to Pharmasset’s closing share price prior to the announcement of the transaction. The acquisition represents a high-risk move for Gilead that hinges significantly upon the success of PSI-7977, Pharmasset’s key Phase III asset.