SLC6A4 and Parkinson disease: Therefore, we retrospectively assessed striatal dopamine transporter (DAT) and midbrain serotonin transporter (SERT) availability as well as the SERT-to-DAT ratios, as measured with [123I]β-carboxymethyoxy-3-beta-(4-iodophenyl) tropane (CIT) (a radiotracer that binds to both the DAT and SERT [18]) single-photon emission computed tomography (SPECT) in drug-naïve early stage PD patients.