SLC6A3 and Parkinson disease: At diagnosis with PD, it is likely that up to 80% of dopaminergic projections to the caudate and putamen have already been lost.44 45 Dopaminergic denervation of the caudal-motor subregion of the striatum, which receives input from the primary motor and premotor cortices, is central to the development of motor symptoms.46 Owing to the degree of early denervation in the caudal-motor region, a floor effect might explain why the relationship between striatal DAT SBR and motor symptoms does not emerge over time following diagnosis.