Factor IX is a vitamin K-dependent plasma protein that participates in the intrinsic pathway of blood coagulation by converting factor X to its active form in the presence of Ca(2+) ions, phospholipids, and factor VIIIa. Selective cleavage of Arg-|-Ile bond in factor X to form factor Xa. Heterodimer of a light chain and a heavy chain; disulfide-linked. Interacts with SERPINC1. Activated by factor XIa, which excises the activation peptide. The propeptide can also be removed by snake venom protease. Belongs to the peptidase S1 family.