Y139H is only more active than wt VKORC1 in the presence of warfarin. (A) The response of VKORC1-dependent fIX carboxylation to warfarin was monitored for wt and Y139H VKORC1. (B) The relative activities were determined by normalizing the fIX carboxylation values to protein levels determined by western analysis with anti-VKORC1 antibody. (C) The same samples were tested in a clotting assay.