Factor XI triggers the middle phase of the intrinsic pathway of blood coagulation by activating factor IX. Selective cleavage of Arg-|-Ala and Arg-|-Val bonds in factor IX to form factor IXa. Inhibited by SERPINA5. Homodimer; disulfide-linked. Forms a heterodimer with SERPINA5. After activation the heavy and light chains are also linked by a disulfide bond. Isoform 2 is produced by platelets and megakaryocytes but absent from other blood cells. N-glycosylated on both chains. N-glycosylated sites mainly consist of nonfucosylated sialylated biantennary (in high abundance) and/or triantennary (in low abundance) complex structures. Glycosylation at Asn-163 uses a rare non-canonical Asn-X-Cys glycosite. Activated by factor XIIa (or XII), which cleaves each polypeptide after Arg-387 into the light chain, which contains the active site, and the heavy chain, which associates with high molecular weight (HMW) kininogen. The disease is caused by variants affecting the gene represented in this entry. Belongs to the peptidase S1 family. Plasma kallikrein subfamily. Factor XI entry Leiden Open Variation Database (LOVD)