HNMT and attention deficit-hyperactivity disorder: In ADHD, the primary common emphasis was on the calcium signalling pathway to a number of metabolic pathways: phenylalanine and tyrosine (DDC and MAOA), tyrosine, histidine (DDC, HNMT, and MAOA) tryptophan (ACAT1, DDC, and MAOA), and unsaturated fatty acid synthesis (FADS1 and FADS2) and to neurotransmitter pathways (cocaine addiction and ligand/receptor interactions).