Swiss-Prot Acc.P51784.Protease that can remove conjugated ubiquitin from target proteins and polyubiquitin chains (PubMed:12084015, PubMed:15314155, PubMed:17897950, PubMed:19874889, PubMed:20233726, PubMed:28992046, PubMed:24724799). Inhibits the degradation of target proteins by the proteasome (PubMed:12084015). Cleaves preferentially 'Lys-6' and 'Lys-63'-linked ubiquitin chains. Has lower activity with 'Lys-11' and 'Lys-33'-linked ubiquitin chains, and extremely low activity with 'Lys-27', 'Lys-29' and 'Lys-48'-linked ubiquitin chains (in vitro) (PubMed:24724799). Plays a role in the regulation of pathways leading to NF-kappa-B activation (PubMed:17897950, PubMed:19874889). Plays a role in the regulation of DNA repair after double-stranded DNA breaks (PubMed:15314155, PubMed:20233726). Acts as a chromatin regulator via its association with the Polycomb group (PcG) multiprotein PRC1-like complex; may act by deubiquitinating components of the PRC1-like complex (PubMed:20601937). Promotes cell proliferation by deubiquitinating phosphorylated E2F1 (PubMed:28992046).