SLC6A4 and Parkinson disease: Studies have shown that 5-HT transporter (SERT) availability is reduced in the caudate-putamen region in human carriers of mutations associated with genetic PD, and the alteration precedes dopaminergic lesions and the onset of motor symptoms, suggesting that 5-HT ergic dysfunction may serve as an early warning of PD (Wile et al., 2017; Wilson et al., 2019).