mRNA expression of the subunit A of the type 3 receptor for 5-hydroxytryptamine (serotonin) (5HT3AR) was significantly up-regulated in patients with PD (Fig. 6d), whereas 5-hydroxytryptamine (serotonin) receptor 4 (5HT4R) was significantly down-regulated (Fig. 6e). The gene discussed is HTR4; the disease is Parkinson disease.