Protease that can remove conjugated ubiquitin from target proteins and polyubiquitin chains. Inhibits the degradation of target proteins by the proteasome. 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). Plays a role in the regulation of pathways leading to NF-kappa-B activation. Plays a role in the regulation of DNA repair after double-stranded DNA breaks. 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. Promotes cell proliferation by deubiquitinating phosphorylated E2F1. Thiol-dependent hydrolysis of ester, thioester, amide, peptide and isopeptide bonds formed by the C-terminal Gly of ubiquitin (a 76-residue protein attached to proteins as an intracellular targeting signal). Monomer (By similarity). Associated component of the Polycomb group (PcG) multiprotein PRC1-like complex (By similarity). Interacts with RANBP9/RANBPM (By similarity). Interacts with BRCA2 (By similarity). Interacts with CHUK/IKKA (By similarity). Interacts with NFKBIA (By similarity). Interacts with SPRY3, RAE1, MYCBP2/PAM, and KCTD6 (By similarity). Predominantly nuclear. Associates with chromatin. Belongs to the peptidase C19 family.