DRD2 and Parkinson disease: However, further studies should be conducted to determine: (i) D2R/M1R heteromer status in human PD striatum (i.e., increase or decrease in the proportion of D2R and M1R protomers forming homomers or heteromers) and (ii) the molecular and functional interplay with other striatal D2R-containing oligomers (i.e., D2R/A2AR heteromers).