Metabolic labeling reveals efficient expression of FVIII B-domain variants in COS-1 cells. COS-1 cells either mock transfected (m) or transiently transfected with FVIII WT, F309S, BDD-FVIII, 226aa/N6, and F309S/226aa/N6 plasmids were pulse labeled for 30 minutes with [³⁵S]-methionine and duplicate plates chased for 4 hours with excess unlabeled methionine. Cell extracts were harvested after pulse labeling (P) and following the 4-hour chase, cell extracts (C) and cell media were harvested. [³⁵S]-methionine-labeled proteins were separated by SDS-PAGE. Molecular-weight markers are indicated on the left. Single chain (SC), heavy chain (HC), and light chain (LC) of FVIII WT within the cell media are indicated on the right for comparison with the FVIII B-domain variants.