FVIII B-domain variants are expressed more efficiently in COS-1 cells. One-stage activity (A) and FVIII antigen (B) for FVIII B-domain variant cDNA constructs transiently expressed in COS-1 monkey kidney cells. One-stage activity was determined by aPTT-based assay. FVIII antigen was determined by a commercial ELISA (Asserachrom FVIII:C Ag). The data were obtained from assaying the conditioned medium at 60 hours following transfection and compared with BDD-FVIII control. Data presented are the mean of several independent experiments (n ≥ 3), and the error bars represent the standard deviation.