Isopeptidase T is a ubiquitin-specific processing protease (UBP) that specifically disassembles free K48-linked polyubiquitin chains into ubiquitin monomers. There are two human isopeptidase T isoforms, Isopeptidase T short form (isoT-S) and Isopeptidase T long form (isoT-L) which differ by an insertion of 23 amino acids in isoT-L. Isopeptidase T is the only UBP known to require zinc binding to be active.