Recently, it was discovered that herpesvirus-associated ubiquitin-specific protease (HAUSP) in human interacts with p53 protein, and removes the ubiquitin from ubiquitinated p53. Thus, human HAUSP stabilizes the status of p53, induces p53-dependent cell growth repression and apoptosis. In this study, we isolated and characterized a mouse orthologue of HAUSP, mHAUSP. The mHAUSP cDNA was cloned from mouse ES cells by RT-PCR. The open reading frame consists of 3,312 bp and encodes a predicted protein of 1,103 amino acids with a molecular weight of approximately 135 kDa. The N-terminal region contains the Cys, His, and Asp domains, which are highly conserved in all deubiquitinating enzymes. Northern blot analysis revealed that two transcripts were detected in various tissues, with strong expression in brain, lung, thymus, and testis. In vivo and in vitro deubiquitinating enzyme assays demonstrated that mHAUSP has deubiquitinating enzyme activity. The overexpression of mHAUSP reduces the amount of ubiquitinated p53, indicating that it functions as a deubiquitinating enzyme for p53.