Thrombin (activated Factor II [IIa]) also commonly called pro-thrombin is a coagulation protein in the blood stream that has many effects in the coagulation cascade. It is a serine protease (EC 3.4.21.5) that converts soluble fibrinogen into insoluble strands of fibrin, as well as catalyzing many other coagulation-related reactions. The Thrombin (prothrombin) gene is located on the eleventh chromosome (11p11-q12). The molecular weight of prothrombin is approximately 72000 gmol^-1. The catalytic domain is released from prothrombin fragment 1.2 to create the active enzyme thrombin, which has a molecular weight of 36000 gmol^-1.There are an estimated 30 people in the world that have been diagnosed with the congenital form of Factor II deficiency, which should not be confused with a mutation of prothrombin.