The present author investigated these candidate SNPs and focused on a common C to T substitution at position 677 of the MTHFR gene (MTHFR C677T polymorphism), which results in an alanine to valine amino acid substitution in the protein and leads to a 30% decrease in enzyme activity in heterozygotes and a 60% decrease in homozygotes.8 This TT genotype was known to cause mild hyperhomocysteinemia more frequently than the major allele homozygote, and its allele frequency approaches 30% in many ethnic groups.9 Here, MTHFR is linked to hyperhomocysteinemia.