ISG15 is an ubiquitin (Ub)-like protein which is conjugated to intracellular proteins via an isopeptide bond. Similar to ubiquitination, the conjugation of ISG15 (ISGylation) requires a three-step process, involving an E1 activating enzyme (UBE1L), an E2 conjugating enzyme (UbcM8/H8), and HERC5/Ceb1 an IFN-inducible ISG15-specific E3 ligase. ISG15 conjugation may play an important regulatory role in IFN-mediated antiviral responses. IFN induces ISG15 conjugation to DDX58 (RIG-I) negatively regulating DDX58-mediated antiviral signaling. ISGylated DDX58 (RIG-I) becomes subject to an irreversible biochemical process, such as proteolysis or proteasomeal degradation.