Clotting factor V is a proenzyme that functions in the blood coagulation cascade. Factor V Leiden (R506Q) mutation is an inherited condition that results in overproduction of the Factor V 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). Symptoms of factor V Leiden vary among individuals. There are some individuals who have the F5 gene and who never develop thrombosis, while others have recurring thrombosis before the age of 30 years. This variability is influenced by the number of F5 gene mutations a person has, the presence of other gene alterations related to blood clotting, and circumstantial risk factors, such as surgery, use of oral contraceptives and pregnancy.

A Factor V Leiden mutation 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 8 fold higher compared to an individual without this mutation. That risk is about 80 fold higher in homozygous carriers, who carry two copies of the mutant gene.