Deubiquitinase with endodeubiquitinase activity that specifically interacts with and cleaves 'Lys-63'-linked long polyubiquitin chains. Shows only weak activity against 'Lys-11' and 'Lys-48'-linked chains. Plays an important role in genome stability pathways, functioning to prevent spontaneous DNA damage and also promote cellular survival in response to exogenous DNA damage. Modulates the ubiquitination status of replication protein A (RPA) complex proteins in response to replication stress. 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). Interacts with RPA1 and RPA2. Mostly present in the nuclear fraction. Localizes to DNA lesions. The motif interacting with ubiquitin (MIU) and ZUFSP ubiquitin-binding domain (zUBD, also called ZUFSP helical arm ZHA) are responsible for binding the distal (outgoing) ubiquitin units S1 and S2 respectively. C2H2-type zinc finger 4 is a ubiquitin-binding zinc finger (UBZ) and required for polyubiquitin binding, possibly binding the proximal ubiqutin, and for catalytic activity. C2H2-type zinc fingers 1-3 are required for localization to sites of DNA damage. Belongs to the peptidase C78 family. ZUFSP subfamily.