A FVIII A1-domain mutation further enhances expression of FVIII B-domain variants in COS-1 cells. One-stage activity (A) and FVIII antigen (B) for FVIII A1-domain/B-domain variant hybrid transiently expressed in COS-1 monkey kidney cells by transfection. One-stage activity was determined by aPTT-based assay. FVIII antigen was determined by 2 different commercial ELISAs, Asserachrom FVIII:C Ag (□) and FVIII:C-EIA (▪). 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.