As previous studies found that high SAM levels act as an allosteric inhibitor of MTHFR (involved in the folate cycle) and activator of CBS, so when cancer cells produce high SAM levels, it prevents Hcy from entering the remethylation pathway rather than allowing excess Hcy to shuttle to the transculturation pathway to produce H2S [71,72,73]. Here, MTHFR is linked to cancer.