Covalent attachment of the C-terminus of ubiquitin to cellular proteins plays a role in a variety of cellular processes. Ubiquitin C-terminal hydrolysis is catalyzed by deubiquitinating (DUB) enzymes and is necessary for several functions, including liberation of monomeric ubiquitin from the precursors encoded by ubiquitin genes and recycling of ubiquitin monomers. There are 2 distinct families of DUBs, ubiquitin-specific proteases (UBPs) and ubiquitin C-terminal hydrolases (UCHs). Mayer and Wilkinson (1989) identified 4 distinct UCH activities from bovine thymus. All 4 were thiol proteases and had high-affinity binding sites for ubiquitin. Wilkinson et al. (1989) purified the predominant isozyme, UCHL3, and raised antibodies against it. By screening a human B-cell expression library with the antibodies, the authors isolated cDNAs encoding human UCHL3. Sequence comparisons revealed that the sequence of the predicted 230aa human UCHL3 protein is 54% identical to that of UCHL1.