ISG15 and infection: At 48 and 72 h after infection, even greater levels of ISG15 expression and protein ISGylation were observed at relatively low MOIs (0.2, 0.5, and 1) (Fig 1A, lanes 13–15 and 24–26); however, the levels of free ISG15 and ISG15 conjugates at high MOIs (3 and 10) were much lower than those at low MOIs (Fig 1A, lanes 16–17 and 27–28).