Arising from: Li, M. et al. Nature416, 648–653 (2002) Ubiquitination of p53 is the principal mechanism through which p53 concentrations in the cell are regulated^, in order to maintain its effects on tumori-genesis and normal cell growth. The protein HAUSP (also known as USP7) is a ubiquitin-specific protease (deubiquitinase)^, that has been shown by Li et al. to bind to p53 (ref. ); in overexpression experiments, Li et al. showed that p53 could be stabilized as a result of deubiquitination by HAUSP and suggested that HAUSP may thereby act as a tumour suppressor^,. Here we use a different approach to investigate the relationship between HAUSP and p53 stability, in which we disrupt the HAUSP gene in human cells by targeted homologous recombination. Instead of the expected increase in ubiquitinated p53 and destabilization of p53, we find that disruption of HAUSP results in the opposite phenotype, leading to stabilization and functional activation of p53 in our system. It may be that HAUSP can deubiquitinate other proteins such as MDM2, another regulator of p53, and that the balance between the deubiquitination of the different targets of HAUSP determines the steady-state level of p53.