This is pronounced 'factor 5 lyden'. It is fairly common in people of European origin - about 1 in 20 Europeans have the factor V Leiden gene. This gene affects the factor V part of the clotting cascade, making the clotting process go on longer. It increases the risk of a blood clot in a vein by about eight times. That is still a relatively low risk, so most people with factor V Leiden do not develop problems. Some people inherit two factor V Leiden genes - one gene from each parent (known as 'homozygous factor V Leiden'). This is less common but makes the risk much higher - around eighty times the normal risk.