A positron emission tomography (PET) study using CB1 receptor selective tracer [18F]MK9470 showed subjects with PD to have a reduction in CB1 availability in substantia nigra compared with healthy control subjects (HCs), and a relative increase of CB1 in nigrostriatal, mesolimbic, and mesocortical pathways and in the putamen contralateral to the most affected body side of subjects with PD.16 This evidence concerns the gene CNR1 and Parkinson disease.