[Isoform 1]: Catalyzes the transfer of a methyl group onto N- acetylserotonin, producing melatonin (N-acetyl-5-methoxytryptamine). . [Isoform 2]: Does not show Acetylserotonin O- methyltransferase activity. . [Isoform 3]: Does not show Acetylserotonin O- methyltransferase activity. . This is the function of ASMT (acetylserotonin O-methyltransferase, Ensembl gene identifier ENSG00000196433).