Factor V Leiden
This gene affects the factor V part of the clotting cascade, making the clotting process go on longer. This increases the risk of a blood clot.
Protein C deficiency
Protein C is a natural anticoagulant chemical in the blood. The deficiency can be genetic, or due to other conditions such as kidney disease. With inherited protein C deficiency, the risk of a blood clot forming varies between families. This is because each family has a different alteration in the protein C gene.
Protein S deficiency
Protein S is also a natural anticoagulant chemical in the blood. Protein S deficiency is rare. The risk of a blood clot forming varies between families.
Antithrombin is another of the natural anticoagulant chemicals in the blood. There are different types of antithrombin deficiency, both genetic and due to other diseases. The inherited form is rare, affecting about 1 in 2,000 people.
The risk of having a blood clot varies between families but can be increased by 25-50 times the normal risk. Others, however, may reach elderly age without problems.
Combined inherited thrombophilias
Some people inherit more than one thrombophilia. With combined thrombophilias, the risk of developing a blood clot is multiplied and there is a much greater risk than with either condition alone.