The murine Mx1 protein was identified as an interferon-inducible host factor that confers the resistance to influenza virus. Mx proteins homologous to the murine Mx1 are subsequently found in a wide variety of species including human, rat, chicken, fish and so on. Mx proteins have the typical GTP-binding motif, and in fact, are shown to be GTP-binding proteins and GTPases. Nuclear Mx proteins selectively inhibit the primary transcription of orthomyxoviruses. In contrast, the cytoplasmic Mx protein, for instance, human MxA blocks the multiplication of not only orthomyxovirus but also rhabdovirus, paramyxovirus, and bunyavirus. Although the molecular mechanism by which Mx inhibits the virus multiplication is not completely clear, Mx may exert the antiviral activity possibly by the mechanism that VPS1p or dynamin, members of the Mx family, is involved in the intracellular protein translocation.