SLC6A3 and Parkinson disease: However, although the positive relationship between striatal DAT and other presynaptic markers of DA system integrity is robust in those with damaged DA neurons due to Parkinson’s or MPTP exposure, it appears much weaker in individuals with intact DA systems or normal levels of DAT25, likely reflecting the fact that there are substantial influences on DAT trafficking26 and DA synthesis27 that are superimposed on individual differences in the number of striatal DA terminals.