Clotting factor II, or prothrombin, is a proenzyme that functions in the blood coagulation cascade. Prothrombin G20210A mutation is an inherited condition that results in overproduction of the prothrombin protein, thereby increasing an individual’s predisposition to develop abnormal blood clots in the veins (deep vein thrombosis or DVT) and lungs (pulmonary embolism or PE).

An abnormal Factor II gene may be inherited from one or both parents. Inheritance of one copy results in heterozygous carrier status, where the risk of developing a deep vein thrombosis or pulmonary embolism is about two to three in 1,000. That risk is several times higher in homozygous carriers, who carry two copies of the mutant gene.