The MTHFR gene codes for the enzyme methylenetetrahydrofolate reductase, which plays an important role in processing amino acids, as it converts the molecule 5,10methylenetetrahydrofolate to 5-methyltetrahydrofolate. This step is required in the process that converts homocysteine to another amino acids, methionine that can be used by the body to make other proteins or compounds. The processing of folic acid is vital for many key processes such as amino acid metabolism, purine and pyrimidine synthesis and methylation of nucleic acids, proteins and lipids. These folate-dependent functions are required for DNA synthesis and repair, control of gene expression and many other biological processes of fundamental importance for cell division and embryo development. Polymorphisms in the MTHFR gene sequence have been associated with subfertility. The most well studies polymorphisms is the C677T polymorphism, which yields a thermolabile MTHFR enzyme with reduced activity. Especially people with two copies of this allele have elevated homocysteine levels in the blood. A second well-known polymorphism is A1298C (rs1801131), which leads to the substitution of Glu429 in the regulatory domain of the human enzyme by an alanine. The mutation is associated with mild reduction in the MTHFR activity and has been suggested to be associated with increased risk for neural tube defects, a higher susceptibility to environmental damage, depression and central nervous system issues. Compound heterozygotes (heterozygous for C677T and A1298C) may develop hyperhomocysteinemia which is a risk factor for coronary artery disease, acute myocardial infarction, peripheral arterial disease, stroke, and venous thromboembolism.