Ubiquitin Carboxyl-Terminal Hydrolase Isozyme L1 (UCHL1) belongs to the Peptidase C12 family. UCHL1 is specifically expressed in the neurons and in cells of the diffuse neuroendocrine system. UCHL1 is a component of the ubiquitin system, which has a fundamental role in regulating various biological activities. UCHL1 is a thiol protease that recognizes and hydrolyzes a peptide bond at the C-terminal glycine of ubiquitin. UCHL1 also binds to free monoubiquitin and may prevent its degradation in lysosomes. The homodimer of UCHL1 may have ATP-independent ubiquitin ligase activity.