Factor V Leiden is a blood clotting disorder. It is not a disease. Factor V is a protein that is needed for blood to clot properly. Some people do not have the normal Factor V protein. Instead, they have an different form called Factor V Leiden. This is caused by a change (mutation) in the gene for this protein.
The different gene that makes the Factor V Leiden protein is inherited from one or both parents. The Factor V Leiden protein is harder to “turn off” than the normal Factor V protein. This makes blood clots more likely to form, a condition called thrombophilia.
If you have Factor V Leiden, you have a greater chance of developing blood clots. Blood clots in the deep veins in your legs (called deep vein thrombosis or DVT) or in your lungs (called pulmonary embolus or PE) can be dangerous.