What you carry at this position in your DNA and what research says about it.
Blood Clot Risk: 10/10 evidence, 74 publications.
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Sample genotype (not yours): C/C
Typical Risk of Blood Clots
Two copies of the common allele result in normal factor V protein function. Your genetic risk for developing abnormal blood clots is at the baseline level for the general population.
What this genotype means: Typical blood clotting function; Baseline risk of venous thromboembolism; Standard response to hormonal contraceptives.
Sample genotype (not yours): C/T
Moderately Increased Risk of Blood Clots
One copy of the Factor V Leiden variant moderately increases your risk of developing blood clots. This risk is further elevated when combined with factors like surgery, long-distance travel, or estrogen-containing medications.
What this genotype means: 3.5x to 4.4x higher risk of deep vein thrombosis; Significantly increased clot risk with oral contraceptives; Increased risk of pregnancy-related clotting complications.
Sample genotype (not yours): T/T
Significantly Elevated Risk of Blood Clots
Two copies of the Factor V Leiden variant significantly increase your risk of developing abnormal blood clots.
What this genotype means: 11x to 18x higher risk of venous thromboembolism; Extremely high clot risk with estrogen-containing medications; Substantially increased risk of recurrent deep vein thrombosis.
Gene function and biological role
Gene Function: The F5 gene encodes coagulation factor V, an essential protein in the blood clotting cascade [1]. Factor V acts as a cofactor that accelerates the conversion of prothrombin to thrombin, the key enzyme that forms blood clots [1]. Under normal conditions, factor V is inactivated by activated protein C (APC) to prevent excessive clotting [2]. When this inactivation process is disrupted, the body remains in a procoagulant state, significantly increasing the risk of abnormal clot formation [2].
Variant Impact: Altering a single amino acid, this missense change is historically known as the Factor V Leiden mutation. It alters a single amino acid in the factor V protein from arginine to glutamine at position 534 (often referred to as position 506 in mature protein numbering) [1]. This specific structural change prevents activated protein C from binding and cleaving factor V [2]. As a result, the altered factor V protein remains active in the bloodstream far longer than normal, promoting continuous thrombin generation [2]. This molecular resistance to inactivation is the primary driver of inherited thrombophilia [3].
Key Insight: Carrying the Factor V Leiden variant can increase the risk of blood clots up to 11-fold, but it historically offered an evolutionary advantage by reducing life-threatening blood loss during childbirth.
Location: F5. Chromosome 1 · 1q24.2 · 169.5 Mb
Origin Story: The Factor V Leiden mutation is believed to have originated in a single individual in Europe approximately 21,000 years ago. It rose in frequency because it provided a critical survival advantage: reducing the risk of lethal blood loss during childbirth and providing protection against severe infections like sepsis.
Estimated prevalence by population
Global 96.6%; European 94.8%; African 99.1%; East Asian 100.0%; South Asian 97.6%; Americas 98.1%; Ashkenazi 96.1%; Finnish 95.4%; Middle Eastern 88.1%; Amish 82.2%.
Estimated CC genotype frequency.
Global 3.4%; European 5.1%; African 0.9%; East Asian 0.0%; South Asian 2.4%; Americas 1.8%; Ashkenazi 3.9%; Finnish 4.6%; Middle Eastern 11.5%; Amish 16.9%.
Estimated CT genotype frequency.
Global < 0.1%; European 0.1%; African < 0.1%; East Asian 0.0%; South Asian < 0.1%; Americas < 0.1%; Ashkenazi < 0.1%; Finnish 0.1%; Middle Eastern 0.4%; Amish 0.9%.
Estimated TT genotype frequency.
Showing effects for C/C genotype
Cardiovascular & Metabolic
Coagulation & Clotting
Venous Thromboembolism. Evidence: 10/10. Source: PMID 22672568. Typical Risk.
Deep Vein Thrombosis. Evidence: 10/10. Source: PMID 26908601. Typical Risk.
Pulmonary Embolism. Evidence: 10/10. Source: PMID 26908601. Typical Risk.
Contraceptive Clot Risk. Evidence: 6/10. Source: PMID 37734636. Typical Response.
Stroke
Ischemic Stroke. Evidence: 10/10. Source: PMID 26908601. Typical Risk.
Blood Cell & Iron Traits
Ferritin Levels. Evidence: 8/10. Source: PMID 33536631. Typical Levels.
Showing effects for C/T genotype
Venous Thromboembolism. Evidence: 10/10. Source: PMID 22672568. ~3.5x to 4.4x Risk.
Deep Vein Thrombosis. Evidence: 10/10. Source: PMID 26908601. Increased Risk.
Pulmonary Embolism. Evidence: 10/10. Source: PMID 26908601. Increased Risk.
Contraceptive Clot Risk. Evidence: 6/10. Source: PMID 37734636. High Clot Risk.
Ischemic Stroke. Evidence: 10/10. Source: PMID 26908601. Slightly Increased.
Ferritin Levels. Evidence: 8/10. Source: PMID 33536631. Slightly Higher.
Showing effects for T/T genotype
Venous Thromboembolism. Evidence: 10/10. Source: PMID 22672568. ~11.4x Risk.
Deep Vein Thrombosis. Evidence: 10/10. Source: PMID 26908601. Highly Increased Risk.
Pulmonary Embolism. Evidence: 10/10. Source: PMID 26908601. Highly Increased Risk.
Contraceptive Clot Risk. Evidence: 6/10. Source: PMID 37734636. Very High Clot Risk.
Ischemic Stroke. Evidence: 10/10. Source: PMID 26908601. Increased Risk.
Ferritin Levels. Evidence: 8/10. Source: PMID 33536631. Higher Levels.
What this genotype means for the people who carry it
Sample genotype: CC (Homozygous)
Inheriting two copies of the common C allele means your body produces the standard version of the factor V clotting protein. This protein is easily regulated and inactivated by activated protein C when clotting is no longer needed [1]. Consequently, you do not have the inherited resistance to activated protein C that characterizes the Factor V Leiden mutation [2]. Your baseline risk for deep vein thrombosis, pulmonary embolism, and other clotting events remains typical [3]. This genotype does not add to the clotting risk of combined hormonal contraceptives or other estrogen-containing medications.
Typical blood clotting function; Baseline risk of venous thromboembolism; Standard response to hormonal contraceptives.
Sample genotype: CT (Heterozygous)
Representing a heterozygous state, this genotype means you are a carrier of the Factor V Leiden mutation, which is present in about 3% to 15% of people of European descent [1]. Your blood clots slightly more easily because about half of your factor V protein resists inactivation by activated protein C [2]. This resistance leads to a 3.5-fold to 4.4-fold increase in your baseline risk for venous thromboembolism and deep vein thrombosis [3]. While most carriers never experience a clot, the risk rises further during prolonged immobility, pregnancy, or surgery [4]. Additionally, taking combined oral contraceptives can multiply this risk significantly [5].
3.5x to 4.4x higher risk of deep vein thrombosis; Significantly increased clot risk with oral contraceptives; Increased risk of pregnancy-related clotting complications.
Sample genotype: TT (Homozygous)
Inheriting the variant T allele from both parents indicates you are homozygous for the Factor V Leiden mutation, meaning all of your factor V protein resists inactivation [1]. This resistance places you in a highly procoagulant state, elevating your risk of venous thromboembolism by approximately 11-fold to 18-fold compared to the general population [2]. The risk of deep vein thrombosis and pulmonary embolism is substantially higher, particularly during periods of immobility, surgery, or trauma [3]. Estrogen-containing medications, such as combined oral contraceptives or hormone replacement therapy, raise this clot risk to an extreme level [4]. Pregnancy also carries a significantly higher risk of obstetric and thrombotic complications [5].
11x to 18x higher risk of venous thromboembolism; Extremely high clot risk with estrogen-containing medications; Substantially increased risk of recurrent deep vein thrombosis.
Variants with shared genetic architecture
rs4524 · F5 Gene · Chr 1 · Blood Clot Risk
rs7545236 · F5 Gene · Chr 1 · Toxin Sensitivity
rs10886430 · GRK5 Gene · Chr 10 · Blood Clot Risk
Mutation in blood coagulation factor V associated with resistance to activated protein C.
Activated protein C resistance caused by Arg506Gln mutation in factor Va.
Association of idiopathic venous thromboembolism with single point-mutation at Arg506 of factor V.
Factor V Leiden and the risk for venous thromboembolism in the adult Danish population.
Factor V Leiden and risks of recurrent idiopathic venous thromboembolism.
Content updated September 2026.
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