The article “Warfarin Therapy: Evolving Strategies in Anticoagulation” (February 1, 1999, page 635) incorrectly stated that the carboxylation of certain glutamic acid residues, which make clotting factors II, VII, IX, and X as well as anticoagulant proteins C and S biologically active, requires an oxidized vitamin K as a cofactor. The article should read, “This carboxylation process requires a reduced vitamin K as a cofactor.” The online version of this article has been corrected.