To test this hypothesis, we used PET imaging and [<sup>11</sup>C]UCB-J-a radiotracer that binds to synaptic vesicle protein 2A (SV2A) to image synaptic density across patients with Parkinson's disease and depressive symptoms (PDd; <i>n</i> = 10), Parkinson's disease patients without depressive symptoms (PDnd; <i>n</i> = 20) and healthy controls (HCs; <i>n</i> = 18). This evidence concerns the gene SV2A and Parkinson disease.