Factor V also known as proaccelerin or labile factor is a crucial protein in the blood coagulation process. It is produced in the liver and circulates in the bloodstream as an inactive precursor, and is later activated by thrombin. Some synonyms for Factor V include coagulation factor V, factor V deficiency, Factor V inhibitor, and Factor V Leiden. Factor V deficiency is a rare genetic disorder where there is a deficiency of factor V in the blood which can lead to excessive bleeding. Factor V inhibitor is a condition where the immune system produces antibodies that attack and destroy Factor V. Factor V Leiden is a genetic mutation that increases the risk of blood clots forming in blood vessels.