SLC6A3 and Parkinson disease: In Parkinson’s Progression Markers Initiative (PPMI) the presence of neuronal α-synuclein is currently determined by a positive cerebrospinal fluid (CSF) α-synuclein seed amplification assay (SAA), and dopaminergic neuronal dysfunction by a single-photon emission computerized tomography (SPECT) dopamine transporter (DAT) scan (DaTscan).