Specific activity of recombinant and transgene-produced FVIII-SQ. (A-B) Recombinant FVIII-SQ (Xyntha) was spiked into plasma from individuals with severe HA, to determine the specific activity using the OS or CS assay. (C-D) Mean specific activity for transgene-produced FVIII-SQ in individual participants from BMN 270 clinical trial 270-201 was determined using the OS or CS assay. (E-F) Mean specific activity for transgene-produced FVIII-SQ in individual participants from BMN 270 clinical trial 270-301 was determined using the OS or CS assay. Dashed lines in panels A, C, and E represent the specification range for specific activity reported for Xyntha (5500-9900 IU/mg).¹⁶  Dashed lines in panels B, D, and F represent the specification range for specific activity reported for ReFacto (7 600-13 800 IU/mg).⁶  Given the lower activity of recombinant FVIII-SQ in the OS than in the CS assay, the specification range depends on which assay is used for product labeling (OS for Xyntha, CS for ReFacto). Error bars in panels C-F represent intraparticipant standard deviations.