SLC6A4 and Parkinson disease: Indeed, recent studies confirm the progressive loss of the serotonin transporter (SERT) in the brainstem in PD patients using DAT-SPECT [82], with significant dysfunction of raphe nuclei in 34% of patients 4 years after diagnosis, and possibly even earlier in SNCA A53T mutation carriers, starting in the premotor phase as measured with 11C-DASB [83].