SLC6A3 and Parkinson disease: For example, the Parkinson's Progression Markers Initiative (PPMI)8 focuses on early, untreated PD subjects, and although it utilizes dopamine transporter imaging (DaTSCAN) to identify subjects with dopaminergic deficits, the cohort may likely include other parkinsonian syndromes that are indistinguishable from PD at that early stage.9 Thus, discovery and validation of biomarkers in a well‐characterized, “typical” (including medically treated and therefore more advanced than the de novo, untreated cohorts) PD population is a critical complementary step for advancing biomarker discovery in PD.