Deficiency of FXI results in a mild to moderate bleeding disorder known as hemophilia C. FXI can be activated by FXIIa29, 30 and thrombin,31, 32 and FXIa has been shown to have numerous natural substrates including FIX, factor V, FX, prochemerin, and complement regulatory protein factor H (FH).33, 34, 35. The gene discussed is F11; the disease is congenital factor XI deficiency.