SLC6A3 and Parkinson disease: Post-mortem studies have shown that motor symptoms in Parkinson’s disease begin at fairly advanced stages of nigrostriatal degeneration, when the loss of dopamine neurons has reached about 50% [64], the depletion of striatal dopamine has reached about 70% [65], and the loss of DAT in the unilateral posterior putamen has reached about 50% [66].