Factor V Leiden thrombophilia is a genetically inherited disorder of blood clotting. Factor V Leiden is a variant (mutated form) of human factor V that causes an increase in blood clotting (hypercoagulability). In this disorder, the Leiden variant (form) of factor V cannot be inactivated (switched off) by activated protein C, and so clotting is encouraged. Factor V Leiden is the most common hereditary hypercoagulability (prone to clotting) disorder amongst European Caucasians.