Mx is an interferon (IFN)-inducible intracellular protein found in various vertebrates that mediates resistance against negative-strand RNA viruses. We have demonstrated previously that feral mouse strains are able to express a functional Mx2 mRNA, while that of the inbred laboratory strains was non-functional because of a single nucleotide insertion in the open reading frame, and under the detectable level. In the present study, we examined the regulation of Mx2 expression in vivo using a congenic mouse carrying Mx1 and Mx2 genes derived from feral strain SPR. Mx2 mRNA was induced strongly in the spleen, ovary and white adipose tissue after the treatment with IFNalpha/beta. Furthermore, we identified the structure of the Mx2 gene. It consists of 14 exons, greatly homologous to the Mx1 gene. The promoter region of Mx2 contained two putative IFN-stimulated response elements (ISREs). We found that the proximal ISRE site positioned between -69 and -55 was essential to IFNalpha/beta-induced transcription by transient transfection assay using reporter gene constructs with mutants of the Mx2 promoter. These results indicate the similarity of the mechanisms of mRNA induction among Mx genes. (C) 2003 Elsevier Science B.V. All rights reserved.