Interferon-stimulated gene 15 (ISG15) is a 17 kDA secreted protein that in humans is encoded by the ISG15 gene. ISG15 is induced by type I interferon (IFN) and serves many functions, acting both as an extracellular cytokine and an intracellular protein modifier. The precise functions are diverse and vary among species but include potentiation of Interferon gamma (IFN-II) production in lymphocytes, ubiquitin-like conjugation to newly-synthesized proteins and negative regulation of the IFN-I response. ISG15-deficiency is a very rare genetic disorder caused by mutations of the ISG15 gene. It is inherited with an autosomal recessive pattern and is classified as a primary immunodeficiency or inborn error of immunity. Patients present in childhood with infectious, neurologic or dermatologic features. Basal ganglia calcification is observed in all patients reported to date and represents the underlying autoinflammatory disease of excessive IFN-I activity, known as type I interferonopathy. The basal ganglia calcifications may cause epileptic seizures but often are asymptomatic. The IFN-I inflammation may also manifest early in life as ulcerative skin lesions in the armpit, groin and neck regions. Finally, ISG15-deficiency leads to mendelian susceptibility to mycobacterial disease, although with incomplete penetrance. These infections present as fistulizing lymphadenopathies and respiratory symptoms following BCG vaccination. In pancreatic ductal adenocarcinoma, tumor-associated macrophages secrete ISG15 enhancing the phenotype of cancer stem cells in the tumor.