Homocysteine is also a precursor of hydrogen sulfide (H2S), which is produced by the transsulfuration pathway where cystathionine β-synthase (Cbs) catalyzes condensation of homocysteine and cysteine forming H2S, and hyperhomocysteinemia is associated with decreased H2S levels [17–19]. The gene discussed is CBS; the disease is hyperhomocysteinemia.