Ubiquitin-like carboxyl-terminal hydrolase 18 (USP18) is a specialized cysteine protease with a predicted molecular weight of 42 kDa.  USP18 from Mus musculus is a member of the peptidase C19 family and shares 70% amino acid identity with its human ortholog.  USP18 plays and important role in antiviral responses and interferon-mediated signaling pathways.  Mice lacking the gene (USP18 ^-/-) are hypersensitive to interferon, have shortened lifespans and pathologic abnormalaties of the brain.  While it was initially speculated that the cause of these phenotypes stems from the accumulaiton of ISG15-conjugated intracellular proteins (a target for USP18 catalytic function), it was later demonstrated that USP18 has multiple roles, at least one of which is independent of its ISGylase activity.