Structural model of FVIII mutations I566T and S558F. (A) Homology model of the triplicated A domains of FVIII is shown as viewed perpendicular to threefold axis with “top” of molecule to top of Fig Red, blue, and green represent A1, A2, and A3 subunit, respectively. Binding loop of factor IXa is shown as magenta CPK spheres. N564 (new N-glycosylation site) and T566 are colored by atom (carbon, green; hydrogen, white; oxygen, red; nitrogen, blue). (B) Factor IXa binding loop 558 to 565 shown in CPK spheres: residue 558 colored by atom, 559 to 565 in gray. Blue ribbons represent the alpha-carbon trace of neighboring residues in the A2 domain. Left, WT S558; Right, variant F558 (reminimized).