PGP9.5/UCH-L1 is a member of a gene family whose products hydrolyze small C-terminal adducts of ubiquitin to generate the ubiquitin monomer. PGP9.5 is a component of the ubiquitin system, which has a fundamental role in regulating various biological activities. PGP9.5 gene encodes two opposing enzymatic activities that affect alpha-synuclein degradation and Parkinson's disease susceptibility.