Transkaryotic Therapies Inc. last week began its defense in the patent infringement trial with Amgen Inc., with the most important testimony coming from Allan Erslev, the scientist who discovered erythropoietin in 1953. Testifying for the defense, the scientist introduced evidence that TKTX believes demonstrates the availability of therapeutically effective EPO before AMGN's filing of its first patent, the Lin patent, in 1984.

A patent cannot be granted on an invention already in existence. If Judge William Young of the U.S. District Court for the District of Massachusetts agrees, the evidence could invalidate the claims of two AMGN patents, Nos. 5,955,422 and 5,547,933.