Severe hyperhomocysteinemia could be caused by cystathionine-β-synthase enzyme deficiency but it could be due to homozygosity of a common 677C/T point mutation in the coding region of the methylenetetrahydrofolate reductase (MTHFR) gene as a 3-fold increase in risk of CAD is associated with the MTHFR 677TT genotype. The gene discussed is MTHFR; the disease is coronary artery disorder.