The INTERFERON SYSTEM provides mammals, birds, and other vertebrates with a powerfultool to defend themselves against infecting viruses. How IFNs block the multiplication of the majority of viruses remains a largely unresolved question. A few of the IFN-induced proteins were shown to function as intracellular mediators of virus resistance, namely the protein kinase PI, the 2', 5'-oligoadenylate synthetase, and Mx proteins (reviewed in Refs. 1-3). In this review we will focus on recent progress toward understanding the antiviralfunctions of Mx proteins from different species. Recent sequence information indicates that Mx proteins belong to a new family of GTP-binding proteins, which includes the yeast protein VPSlp/SP015 and the rat protein dynamin.'4'Both proteins VPSlp/SPO15 and dynamin serve basic cellular functions and appear to be involved in active microtubule-dependent intracellular transport.< 5_7'These findings would seem to indicate that Mx proteins exert their antiviral functions via similar mechanisms.