SLC6A3 and diabetes mellitus: DPP-4 inhibitors were found in a retrospective study to have a beneficial effect in patients with DM and PD (n = 54); these patients had higher long-term motor performance and greater baseline dopamine transporter availability than did patients with PD but without DM (n = 558) or patients without DM who did not receive a DPP-4 inhibitor (n = 85) did [35].