HCY (adenosylhomocysteinase), also known as S-Adenosylhomocysteine hydrolase (SAHH), is the only enzyme that catalyzes the reversible hydrolysis of S-adenosylhomocysteine (SAH) to adenosine and homocysteine. AHCY (SAHH) is involved in the regulation of intracellular methylation processes because SAH acts as an inhibitor of various S-adenosylmethionine-mediated methylation reactions such as methylation of DNA, RNA and protein.