Rituxan - $3 billion


2011 sales: $3 billion
Developer: Genentech in concert with IDEC (pre-merger with Biogen)
FDA approvals: Non-Hodgkin lymphoma, 1997; rheumatoid arthritis symptoms, 2006; rheumatoid arthritis disease progression, 2008; chronic lymphocytic leukemia, 2010; Wegener's granulomatosis and microscopic polyangiitis, 2011

Rituxan owns the distinction of being the first monoclonal antibody treatment for cancer. Before it, the use of such drugs--which has become so prevalent--was relatively murky. Since, Rituxan's success has transformed cancer treatment, clearing the way for other monoclonal antibody treatments, including its Genentech sisters Campath and Avastin.

Even after 14 years on the market, Rituxan continues to grow. Its 2011 sales marked a 9% increase over 2010, and its $2.76 billion in 2010 revenue outpaced 2009 by 5%. That's partly because Genentech has managed to broaden its market repeatedly. Originally approved for a subset of non-Hodgkin lymphoma patients, Rituxan has racked up several other lymphoma indications since, as well as approvals for rheumatoid arthritis, a sizable market on its own.

Rituxan - $3 billion