Despite constituting the most common genetic cause of mild to moderate hyperhomocysteinemia, homozygosity for the MTHFR 677T allele, which leads to the substitution of valine by alanine in a potential folate-binding site and thermolability of the enzyme, has not been clearly associated with atherothrombotic disease (107). The gene discussed is MTHFR; the disease is hyperhomocysteinemia.