Guanylate-binding proteins, GBP1 and GBP2 are GTP-binding proteins with a high-turnover GTPase activity and an antiviral effect (1-4). GBP1 mediates an antiviral effect against vesicular stomatitis virus and encephalomyocarditis virus and plays a role in the IFN-mediated antiviral response against these viruses (4). GBP1 and GBP2 belong to a group of large GTP-binding proteins with a high concentration-dependent GTPase activity that have the common ability to undergo oligomerization (1). GBP1 and GBP2 are bone marrow-derived GTPases encoded by interferon-activated genes and are inducible following IFN treatment (2,3). Specifically, GBP1 is expressed in cultured mammary epithelial tumor cell lines after treatment with IFN-gamma and LPS.