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). Here, PRKN is linked to viral infectious disease.