CTH and Down syndrome: Deficiency in the expression of enzymes such as cystathionine β-synthase (CBS) and cystathionine γ-lyase (CSE or CGL) or their cofactors may lead to high levels or abnormal accumulation of Hcy, which characterizes inherited diseases such as homocystinuria [22], Down syndrome [23,24] and other clinical conditions such as vitamin (folate [25], cobalamin (vitamin B12) [26] or vitamin B6 [27]) deficiency, cardiovascular disease [28] and renal failure [29].