ISG15 and cytomegalovirus infection: In our analysis, however, overexpression of ISG15GG with UBE1L, UbcH8, and Herc5 led to a mild reduction of HCMV growth, whereas ISG15AA, an inactive form, did not significantly affect viral growth, suggesting that expression of free ISG15 prior to HCMV infection may minimally affect viral growth in cultured cells.