USP18 and neoplasm: To detect ISG15 in the tumor microenvironment, tumor lysate was prepared from B16-OVA-GFP, B16-OVA-USP18 or B16-OVA-shUSP18 tumors, and ELISA or western blot analysis showed that the ISG15 level was higher in B16-OVA-USP18 tumor than in B16-OVA-GFP tumor, but lower in B16-OVA-shUSP18 tumor (Figure 4E, Additional file1: Figure S6).