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F9 gene
Reviewed by Medicinsk Expert•Last updated: 2026-10-22•Gene

F9 gene

The F9 gene provides instructions for making coagulation factor IX, a protein essential for the formation of blood clots to prevent excessive blood loss.

Normal Function

The F9 gene provides instructions for making a protein called coagulation factor IX. Coagulation factors are a group of related proteins that are essential for the formation of blood clots. After an injury, clots protect the body by sealing off damaged blood vessels and preventing further blood loss.

Coagulation factor IX is made in the liver. This protein circulates in the bloodstream in an inactive form until an injury that damages blood vessels occurs. In response to injury, coagulation factor IX is activated by another coagulation factor called factor XIa. The active protein (sometimes written as coagulation factor IXa) interacts with coagulation factor VIII and other molecules. These interactions set off a chain of additional chemical reactions that form a blood clot.

Health Conditions Related to Genetic Changes

Hemophilia

Mutations in the F9 gene cause a type of hemophilia called hemophilia B. These mutations lead to the production of an abnormal version of coagulation factor IX or reduce the amount of this protein, preventing blood from clotting properly. Mutations that completely eliminate the activity of coagulation factor IX result in severe hemophilia, while those that reduce activity usually cause mild or moderate hemophilia.

A specific form known as hemophilia B Leyden involves mutations near the beginning of the F9 gene sequence. People with these mutations are born with very low levels of functional coagulation factor IX, but hormonal changes during puberty cause the levels of this protein to increase, meaning adults with this condition rarely experience abnormal bleeding.

Warfarin sensitivity

Several rare mutations in the F9 gene cause an increased sensitivity (hypersensitivity) to a drug called warfarin. Warfarin is an anticoagulant used to prevent the formation or growth of abnormal blood clots by reducing the amount of active factor IX and other coagulation proteins. People with these specific mutations do not have hemophilia B, but they are at risk of recurrent, severe bleeding problems if treated with warfarin.

Frequently Asked Questions

Hemophilia B is caused by mutations in the F9 gene affecting factor IX, whereas hemophilia A involves factor VIII. Both present with joint and muscle bleeding, but hemophilia B may have a slightly longer bleeding time. Treatment is similar: replacement therapy with factor IX concentrates or recombinant factor IX, but dosing and half-life may differ.
Gene therapy trials using adeno-associated virus (AAV) vectors to deliver a functional F9 gene have shown sustained factor IX expression and reduced bleeding. Some patients have achieved near-normal factor IX levels for months to years. Availability depends on regulatory approval and clinical trial enrollment, but several programs are currently recruiting.
Bleeding can be triggered by trauma, surgery, dental work, or even spontaneous joint bleeds. Prevention includes prophylactic factor IX infusions, especially in patients with moderate to severe disease, and avoiding high-impact activities. Patients should also use protective gear and monitor for signs of bleeding.
Hemophilia B Leyden involves mutations near the start of the F9 gene that severely reduce factor IX levels at birth. During puberty, hormonal changes (particularly increased estrogen) upregulate factor IX production, raising levels into the mild range and reducing bleeding frequency in adulthood.
Source: MedlinePlus, National Library of Medicine

MedlinePlus brings together authoritative health information from the National Library of Medicine (NLM), the National Institutes of Health (NIH), and other government agencies and health-related organizations.

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