Description: This gene encodes coagulation factor XI of the blood coagulation cascade. Alternative splicing of the gene results in two transcripts. The full length transcript encodes the circulating plasma factor XI and an alternate transcript lacking exon 5 encodes the platelet factor XI. The plasma factor XI is present in plasma as a zymogen, which is an unique plasma coagulation enzyme because it exists as a homodimer consisting of two identical polypeptide chains linked by disulfide bonds. During activation of the plasma factor XI, an internal peptide bond is cleaved by factor XIIa (or XII) in each of the two chains, resulting in activated factor XIa, a serine protease composed of two heavy and two light chains held together by disulfide bonds. This activated plasma factor XI triggers the middle phase of the intrisic pathway of blood coagulation by activating factor IX. Defects in this factor lead to Rosenthal syndrome, a blood coagulation abnormality. The platelet factor XI is localized to platelets and megakaryocytes but absent from other blood cells. The nature of this platelet factor is not known, it may play an important role both in the maintenance of normal hemostasis and as a substitute for plasma factor XI.