USP7 (HAUSP) is able to deubiquitinate many substrates. It is a key regulator of the tumor suppressor TP53 (p53) (Vousden & Lu 2002). It can act on TP53 directly, or indirectly by acting on the E3 ligase MDM2, which can ubiquitinate TP53 (Chene 2003, Li et al. 2002, 2004, Kon et al. 2010). USP7 also regulates MDM4 (Mdmx), a structural homolog of MDM2 (Meulmeester et al. 2005, Chen 2012). USP7 interacts with and deubiquitinates FOXO4 in response to oxidative stress (van der Horst et al. 2006) and reduces monoubiquitinylation of PTEN, presumably on the previously identified lysine residues 13 and 289 (Trotman et al. 2007), reducing nuclear PTEN levels (Song et al. 2008).