Protease that can remove conjugated ubiquitin from target proteins and polyubiquitin chains(PubMed:12084015, PubMed:15314155, PubMed:17897950, PubMed:19874889, PubMed:20233726, 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).