SLC6A3 and Parkinson disease: As reported previously in this sample, DAT specific binding ratio (SBR) in participants with PD was, on average, around half that of healthy controls, and as expected, there was evidence of a marked decline over time (mean ± standard deviation (s.d.)percentage reduction from baseline: year 1, −9.7 ± 17.4%; year 2, −16.6 ± 17.7%; year 4, −26.6 ± 18.4%)16.