Plasma elevation of total homocysteine (tHcy) or hyperhomocysteinemia may result from congenital deficiency of cystathionine β-synthase (CBS) leading to homocystinuria, or more frequently from polymorphisms of the cystathionine γ lyase (CTH) gene (OMIM *607657; EC 4.4.1.1). This evidence concerns the gene CBS and hyperhomocysteinemia.