TPH1 and brain disorder: Tryptophan hydroxylase (TPH), the rate-limiting enzyme for serotonin synthesis, exists in 2 forms: TPH1, which is primarily in the gut, and TPH2, which is dominant in the brain and gut neurons.[61] The gut microbiota affects the synthesis and degradation of neurotransmitters such as serotonin, influencing mood regulation, and indicating that serotonin is a key factor in the pathophysiology of gut–brain diseases.[60]