ISG15 and viral infectious disease: Previous studies have shown that IFN-α treatment of ISG15-deficient patient cells renders them resistant to viral infection by several viruses, including the murine respirovirus (family Paramyxoviridae, subfamily Orthoparamyxovirinae) Sendai virus (14), and this seems to extend to PIV5 with our in vitro system (Fig. 2B, 2C).