Factor V Leiden

Factor V Leiden is a point mutation, meaning the change of a single letter, in the F5 gene. That gene holds the instructions for making a protein called coagulation factor V, one of the pieces the blood uses to form clots. The result of the mutation is a thrombophilia: a greater than normal tendency to form abnormal clots that can block a vessel.

To understand why, you have to look at the brake and not the accelerator. In a person without the variant, a protein called activated protein C inactivates factor V, and that slows the coagulation process and keeps clots from growing too large. In someone who has factor V Leiden, that protein can no longer inactivate it. The brake is still there, but it no longer bites.

Now the part that almost always gets told wrong. Having the variant is not having the disease: only about ten percent of the people who carry it ever go on to form an abnormal clot.