Another variant (the thermolabile C677T variant) in the gene MTHFR (Entrez Gene ID 4524), encoding methylenetetrahydrofolate reductase, predisposes heterozygous carriers to hyper-homocysteinemia and may have a synergistic effect on risk for recurrent venous thromboembolism [21], [22]. This evidence concerns the gene MTHFR and hyperhomocysteinemia.