In 1991, NeXstar Pharmaceuticals was founded in Boulder, Colo. through the merger of NeXagen and Vestar, Inc. The new company was engaged in the discovery, development, manufacture, and commercialization of products to treat serious and life-threatening illnesses. NeXstar had research, development, and manufacturing facilities in San Dimas, CA, and marketing subsidiaries worldwide.

On Jul. 29, 1999, Gilead Sciences and NeXstar announced the closing of their agreement whereby Gilead acquired NeXstar in an all-stock transaction valued at $550 million in Gilead Sciences stock.