Correct epigenetic regulation is essential to cellular development, and methyltransferases are enzymes important for epigenetic regulatory processes. They add methyl groups to their substrates, which can be DNA, proteins, or small-molecule secondary metabolites. Methyltransferases have been implicated in a number of diseases, including cancer, HIV infection, and diabetes, yet many remain uncharacterized.S-adenosyl methionine (SAM) is used as a methyl group donor by a majority of methyltransferases. Use of SAM by a methyltransferase results in the production of S-adenosyl homocysteine (SAH). SAM is found across all branches of life, and therefore represents a useful biological marker for methyltransferase activity. Researchers at UC Berkeley have developed a sensitive and selective means of assaying methyltransferase activity. This assay monitors the presence of SAH, and can be used for high-throughput screening.