Cystathionine-β-synthase (CBS) deficiency, also known as classical homocystinuria, is an autosomal recessively inherited metabolic disorder that results in accumulation of homocysteine (Hcy) and methionine (Met) but, contrarily, depletion of cystathionine and cysteine due to mutations in the CBS gene. The gene discussed is CBS; the disease is Other metabolic disease.