ISG15 and viral infectious disease: When co-IP assays were performed using HCMV (Towne)-infected cell lysates, immunoprecipitation with an anti-UL26 antibody co-precipitated unconjugated free ISG15 (Fig 5E), and in a reciprocal experiment, immunoprecipitation with an anti-ISG15 antibody co-precipitated UL26-p21 (Fig 5F), indicating that UL26-p21 interacts with ISG15 during virus infection.