The ubiquitin (Ub) pathway involves three sequential enzymatic steps that facilitate the conjugation of Ub and Ub-like molecules to specific protein substrates. Through the use of a wide range of enzymes that can add or remove ubiquitin, the Ub pathway controls many intracellular processes such as signal transduction, transcriptional activation and cell cycle progression. Ubiquitin-specific-processing protease 19 (USP19), also known as ubiquitin carboxyl-terminal hydrolase 19, ubiquitin thioesterase 19 and deubiquitinating enzyme 19, is a 1318 amino acid member of the peptidase C19 family of proteins. USP19 is thought to be involved in the ubiquitin-dependent proteolytic pathway in conjunction with the 26S proteasome. Four known isoforms of USP19 exist as a result of alternative splicing events.