PSMA5 and Parkinson disease: Of particular interest is the presence of 10 proteins of the 20S Proteasome complex (PSMA1-3, PSMA5-7, PSMB1, PSMB3, and PSMB5-6), which is a part of the ubiquitin proteasome system (UPS), known to be impaired in PD and the major cause of neuronal degeneration [44].