As ISG15 is strongly induced by various types of external stimuli, such as virus infection, LPS, genotoxic stresses and retinoic acid [42–45], we then investigated whether ISG15 conjugation to parkin occurs in response to other selected signals, such as LPS, camptothecin (CPT) and etoposide (Eto). This evidence concerns the gene ISG15 and viral infectious disease.