DRD2 and Parkinson disease: The evidence of striatal D2R reserve in humans is indirect and based on findings that greater than >80% D2R blockade is required for antipsychotic efficacy36 and that motor symptoms with Parkinson disease do not emerge until striatal DA concentrations are lowered by approximately 80%37, which indicates that relatively low levels of dopamine signaling can sustain motor function whereas excessive signaling can trigger pathology.