All considering, we suppose that the reduced 5-HT transporter availability, as shown by Qamhawi and collegues, and the 5-HT synthesis, metabolism, and turnover impairment, resulting in reduced CSF levels of 5-HT and 5-HIAA (8) may both contribute to the serotonergic transmission dysfunction evident in PD. Here, SLC6A4 is linked to Parkinson disease.