 The more common genetic thrombophilias were described in the 1990s . Many studies had previously indicated that many people with thrombosis showed resistance activated protein C. In 1994 a group in Leiden , The Netherlands , identified the most common underlying defect — a mutation in factor V that made it resistant to the action of activated protein C. The defect was called factor V Leiden , as genetic abnormalities are typically named after the place where they are discovered . Two years later , the same group described a common mutation in the prothrombin gene that caused elevation of prothrombin levels and a mild increase in thrombosis risk . 
