ISG15 and Mendelian susceptibility to mycobacterial diseases: Besides the antiviral activity of ISG15, it has been recently described that a mutation in human ISG15 correlates with “Mendelian susceptibility to mycobacterial disease” (MSMD), a rare disorder that manifests in severe clinical symptoms following infection with weakly virulent mycobacterial strains and other intracellular pathogens [28].