Factor VIII (FVIII) is a heterodimer consisting of a heavy chain (ranging in mass from 90 to 200 kDa) bound via metal ions to a light chain (80 kDa). In plasma, FVIII circulates in an inactive form bound to von Willebrand factor. Following activation by factor Xa or thrombin, factor VIIIa can function as cofactor for the enzyme factor IXa in the activation of factor X in the presence of phospholipid and Ca2+. Absent or defective FVIII is the cause of the X-linked recessive bleeding disorder hemophilia A. GMA-8011 (also known as 2A9) recognizes the C1 domain of FVIII, inhibits FVIII activation and binding to VWF and phospholipids, and displays subnanomolar binding to FVIII by surface plasmon resonance.