ISG15 (ISG15 ubiquitin-like modifier), a ubiquitin-like protein, is one of the major type I IFN (interferon) effector systems. ISG15 can be conjugated to target proteins (ISGylation) via the stepwise action of E1, E2, and E3 enzymes. Conjugated ISG15 can be removed (deISGylated) from target proteins...