Unique among coagulation factors, the coagulation factor (F) XI arose through a duplication of the gene KLKB1 that encodes for plasma prekallikrein. This evolutionary origin sets FXI apart structurally as it is a homodimer with two identical subunits comprised of 4 apple and 1 catalytic domain. Each domain exhibits unique affinities for binding partners within the coagulation cascade, regulating the conversion of FXI to a serine protease as well as the selectivity of substrates cleaved by …