Endothelial cells are known to be particularly vulnerable to hyperhomocysteinemia, since they do not express CBS, the first enzyme of the hepatic reverse transsulfuration pathway, or betaine-homocysteine methyltransferase (BHMT), which catalyzes the alternate remethylation pathway in the liver using betaine as a substrate [51]. The gene discussed is CBS; the disease is hyperhomocysteinemia.