Ubiquitin carboxyl-terminal hydrolase isozyme L1 (UCHL1) ,a member of the peptidase C12 family, is also known as neuron cytoplasmic protein 9.5 (PGP 9.5) and ubiquitin thioesterase L1.About 30% of total UCHL1 is associated with membranes in brain. UCHL1 is involved both in the processing of ubiquitin precursors and of ubiquitinated proteins. This enzyme is a thiol protease that recognizes and hydrolyzes a peptide bond at the C-terminal glycine of ubiquitin. Also,UCHL1 can bind to free monoubiquitin and may prevent its degradation in lysosomes. The homodimer of UCHL1 may have ATP-independent ubiquitin ligase activity. Furthermore, the mutations of UCHL1 gene can result in parkinson disease 5.