SLC6A3 and Parkinson disease: Several key categories of exosome biomarkers have been identified for PD: (i) α-synuclein-related markers: alpha-synuclein, Lewy body, etc.; (ii) neurotransmitter-related markers: dopaminergic neuron, dopamine transporter, etc.; (iii) inflammation- and immune system-related markers: TNF alpha, cytokine, etc. [2]; (iv) Alzheimer’s disease-related markers: tau, presenilin, etc. [21]; (v) miRNA biomarkers: downregulation of miR-1, miR-22, and miR-29a, as well as the upregulation of miR-16-2, miR-26a-2, and miR-30a, in PD patients [22].