Some of the alterations reported in the serotonergic system in patients with a PD diagnosis include reduced availability of the serotonin (5-HT) transporter (SERT) and the 5-HT receptor 1A in the striatum [7,8], reduction in striatal levels of both 5-HT and its metabolite 5-hydroxy indole acetic acid, and reduced expression of the rate-limiting enzyme in 5-HT biosynthesis, tryptophan hydroxylase (TPH) [9,10]. This evidence concerns the gene SLC6A4 and Parkinson disease.