ISG15 and hepatocellular carcinoma: ISG15 can also be secreted from activated cells as free ISG15 and has been shown to play putative roles in other solid tumors, such as pancreatic ductal adenocarcinoma, hepatocellular carcinoma, and nasopharyngeal carcinoma (Burks et al. 2019; Chen et al. 2020; Li et al. 2014).