SLC6A3 and Parkinson disease: The first of these tracers which demonstrated its potency in the field of PD using SPECT imaging was the 2β-carbomethoxy-3β-(4-iodophenyl)tropane (β-CIT) (15), which bound to the DAT with a high affinity (around 3 nM) and accumulated significantly in dopaminergic brain areas when labeled with iodine-123.