In biotech, one drug can make a company. For Amgen of Thousand Oaks, Calif., the nation's largest biotech firm, that drug is the anemia treatment erythropoietin (EPO). The best-selling protein racked up $2 billion in revenues for Amgen in 1999 and is paying for the 87,000-square-meter (285,000-square-foot), state-of-the-art laboratory the company is building in the heart of Cambridge, Mass.

But just a few blocks away, at Transkaryotic Therapies (TKT), some clever Cambridge competitors are set to challenge Amgen's pre-eminence with a version of EPO they believe gets around Amgen's suite of patents. If successful, TKT's "end-around" could lead to additional knockoffs of some of the world's largest-selling biotech pharmaceuticals.