Mx proteins are one of the key components of the antivirals induced by interferons in many species. They are a class of dynamin-like GTPases and share structural and functional properties with dynamin, such as self-assembly and association with intracellular membranes. A unique property of some Mx proteins is their antiviral activity against a wide range of viruses. These viruses are inhibited at an early stage in their life cycle, soon after the entry into the host cell and before their genome amplification. It is known now that some Mx proteins recognize the viral nucleocapsid components and interfere with their role in viral genome replication. Crystal structures of some members of dynamin-like GTPase have been solved. Solution of the crystal structures of Mx proteins is of major significance for not only understanding their antiviral function but also prophylaxis and control of the emerging and re-emerging viruses.