K63-linked polyubiquitin chains containing more than two ubiquitin moieties, but not ubiquitin monomers or K48-linked chains, could potently activate the N-terminal portion of RIG-I in vitro.

They cleaved specifically polyubiquitin chains called K63-linked, which is one of eight flavors of ubiquitin chains. "This was surprising, as no other human USP enzyme shows such preference for a ...