Interferon (IFN) regulates various target genes that mediate important roles in immune responses such as antiviral state. Protein ISG15-conjugation (ISGylation) is implicated in the IFN-induced antiviral response. Here we demonstrate that Efp mRNA as well as protein expression could be up-regulated by Type I IFN in HeLa cells and HepG2 cells. Luciferase assay reveals that the first intron of Efp gene contains a functional interferon-stimulated response element (ISRE) and electrophoretic mobility shift assay shows that the ISRE binds to signal transducer and activator of transcription 1 (STAT1). Chromatin immunoprecipitation assays have shown that the ISRE recruits STAT1 in vivo IFN-dependently. Moreover, we demonstrate that Efp protein could be conjugated with not only ubiquitin but also ISG15. These data suggest that Efp is an IFN-responsive gene that potentially mediates IFN actions, involved in ISGylation and ubiquitination of proteins including Efp itself. (c) 2006 Elsevier Inc. All rights reserved.