Recombinant-derived ^ARG531^HISdemonstrates a similiar phenotype to described^ARG531^HIS patient plasmas. Plasma activity data were derived from the HAMSTeRS hemophilia A mutation database for all reported patients identified with ^ARG531^HIS for which both one-stage (▪) and two-stage (□) results were available. The data from recombinant-derived ^ARG531^HISwere obtained from assaying the activity in the conditioned medium from four independent transfection experiments. Activity for^ARG531^HIS patient plasmas and recombinant-derived ^ARG531^HIS is presented as the percentage of wild-type (normal plasma or recombinant FVIII WT, respectively).