Enables ISG15-specific protease activity. Acts upstream of or within response to bacterium and response to stilbenoid. Predicted to be located in intracellular membrane-bounded organelle. Predicted to be active in cytosol and nucleus. Is expressed in axial skeleton; lung; and mandible. Orthologous to several human genes including USP18 (ubiquitin specific peptidase 18). [provided by Alliance of Genome Resources, Apr 2022]