– HNMT (Histamine N-methyltransferase) is a Protein Coding gene. This gene encodes the first enzyme which is found in the cytosol and uses S-adenosyl-L-methionine as the methyl donor. HNMT, the major enzyme for the metabolism of histamine in the rat brain, is potently inhibited by 9-amino-1,2,3,4-tetrahydroacridine (tacrine). Methylation is an important pathway in the biotransformation of many drugs, neurotransmitters, and xenobiotic compounds. Histamine N-methyltransferase (HNMT) catalyzes the N tau-methylation of histamine and structurally related compounds. Histamine N-methyltransferase (HNMT) is believed to be the sole pathway for termination of the neurotransmitter action of histamine in the mammalian brain. That highlights the importance of the inclusion of HNMT for genetic testing of individuals presenting with intellectual disability.