In late April, the US District Court of Massachusetts ruled that the erythropoietin (EPO) produced and used by Transkaryotic Therapies (TKT; Cambridge, MA) and its partner Aventis Pharma (Frankfurt) is covered by a claim in one of Amgen's (Thousand Oaks, CA) five EPO patents. After hearing Amgen argue that the “pharmaceutical composition” of TKT's 165-amino acid EPO was chemically and functionally indistinguishable from its own 166-amino acid version, chief Judge William Young made a summary judgment of infringement of Amgen's US Patent 5,555,422. Although the ruling limits TKT's defense strategy with respect to that patent (Nat. Biotechnol. 18, 483), issues of patent infringement, validity, and enforcement for the other four patents are still being argued at the trial that began in mid-May.