FVIII B-domain variants are expressed more efficiently in CHO cells. Both 1- and 2-stage activity (A) and FVIII antigen (B) for FVIII B-domain variant cDNA constructs transiently expressed in CHO cells. One-stage activity (□) was determined by aPTT-based assay; 2-stage activity (▪) was determined by a commercial chromogenic assay (COAMATIC). 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.