Factor XI is a key player in the intrinsic blood coagulation pathway. It circulates in complex with high molecular weight kininogen. When activated by factor XIIa, thrombin or autoactivation, the activated form factor XIa contributes to both hemostasis and thrombosis by amplifying thrombin generation and fibrin formation leading to clot formation. Human factor XI has a molecular weight of 160 kDa, and is comprised of a disulfide-linked dimer, with each monomer containing four apple domains that form a ring around the base of the catalytic domain.