SLC6A3 and Parkinson disease: These results obtained via a complementary approach further confirm: (1) there is a communication between CNS and peripheral immune cells, (2) CNS dopamine depletion underlies increased DAT+/TH+ PBMCs, and importantly, (3) treatment targeted towards replenishing brain dopamine restores peripheral DAT+/TH+ PBMCs, further validating the sensitivity of these markers for PD.