Classical homocystinuria (MIM# 236200), also known as homocysteinemia type 1, is a rare congenital metabolic disorder caused by deficiency of cystathionine β-synthase (CBS), an enzyme that catalyzes the transsulfuration of homocysteine (Hcy) to cystathionine with the help of its cofactor pyridoxine [1]. The gene discussed is CBS; the disease is homocystinuria.