Genetic risk factors are involved in the predisposition of individuals to venous thrombosis. The most common mutation associated with inherited thrombosis is Factor V Leiden G1691A mutation and results in resistance to activated protein C. The second most common mutation associated with hereditary thrombosis is the G20210A mutation in the prothrombin (Factor II) gene. Increased plasma homocysteine level is another risk factor for venous thrombosis, and is associated with homozygosity for a genetic variant in the methylene tetra-hydrofolate reductase (MTHFR) gene. The MTHFR gene produces an enzyme that helps regulate homocysteine levels in the body by catalyzing the conversion of 5,10-methylene-tetrahydrofolate (5,10-methylene-THF) to 5-methyltetrahydrofolate (5-methyl-THF), which can methylate homocysteine to form methionine. The mutated C677T MTHFR gene results in a thermolabile MTHFR, with reduced activity, resulting in a higher level of homocysteine in the blood. The higher homocysteine levels may cause irritation of the blood vessels. Elevated levels of homocysteine show an increased risk for atherosclerosis, which could eventually result in a heart attack and/or stroke, and blood clots in the veins, referred to as venous thrombosis. Individuals with two copies of 677C (677CC) have the most common genotype. 677TT individuals (homozygous) have lower MTHFR activity than CC or CT (heterozygous) individuals. To have any detrimental effect, mutations must be present in both copies of a person’s MTHFR genes. The variant is commonly found in North American, European, and Asian populations, with the highest frequencies among Southern Mediterranean and Hispanic populations in North America.