TPH1 and Anxiety: A number of genes related to the serotonin neurotransmitter pathway, including the monoamine oxidase A and B (MAOA and MAOB), serotonin transporter (SLC6A4), tryptophan hydroxylases 1 and 2 (TPH1 and TPH2), catechol-O-methyl transferase (COMT), and dopamine-beta-hydroxylase (DBH) have been linked to anxiety and stress response in pigs [11].