SAMe (as S-adenosyl-L-methionine) is a naturally occurring metabolite found in the human body and in plant and animal foods. It is the most active of all methyl donors and has been compared to adenosine triphosphate (ATP) in its importance for the body. SAMe is naturally synthesized in humans from the amino acid methionine in the presence of the cofactors vitamin B12 and folate. SAMe is involved in the synthesis of neurotransmitters, the hormone melatonin, phospholipids, and polyamines, which control cellular growth. It is also the source of methyl groups inside the nucleus for DNA methylation, which controls gene expression and the masking of genetic damage.