Interferon-induced GTP-binding protein Mx1 is a protein that in humans is encoded by the MX1 gene. This cytoplasmic protein is a member of both the dynamin superfamily and the family of large GTPases. MX1 is an antiviral protein that confers resistance to RNA viruses. MX1 has GTPase activity, and GTP-bound MX1 adopts a conformation that enables interaction with viral nucleocapsids. This interaction blocks transport of viral nucleocapsids to the nucleus, which prevents transcription of the viral genome. Structural studies suggest that the antiviral activity of MX1 involves the formation of MX1 oligomeric rings around viral nucleocapsids. In mice, the interferon-inducible Mx protein is responsible for a specific antiviral state against influenza virus infection.