HTR4 and bipolar disorder: In addition to COMT, bipolar disorder has been associated with polymorphisms in a number of other genes coding for monoamine receptors, transporters, and synthetic and catabolic enzymes, including monoamine oxidase (MAOA), dopamine transporter (DAT), serotonin transporter (5HTT), tryptophan hydroxylase (TPH2), and the D2, D4, 5HT4, and 5HT2A receptors (42–45).