Inactivates histamine by N-methylation. Plays an important role in degrading histamine and in regulating the airway response to histamine. histamine + S-adenosyl-L-methionine = H(+) + N(tau)-methylhistamine + S-adenosyl-L-homocysteine Monomer. Variant Ile-105 has a reduced activity and seems to be linked with a predisposition to asthma. The disease is caused by variants affecting distinct genetic loci, including the gene represented in this entry. Has no histamine-methylating activity. Belongs to the class I-like SAM-binding methyltransferase superfamily. HNMT family.