DRD2 and Parkinson disease: Five studies, with 2668 cases and 4620 controls, examined polymorphisms in the dopamine D2 receptor (DRD2) gene, with Taq1A (rs1800497) and Taq1B (rs1079597) being the commonest to be tested, and consistently found no modulation of DRD2 polymorphisms on the smoking effect on PD risk across diverse ethnicities and genders [13,23,25,29,35].