Anthrax infection is initiated by the inhalation, ingestion, or cutaneous contact with Bacillus anthracis endospores. B. anthracis produces three polypeptides that comprise the anthrax toxin: protective antigen (PA), lethal factor (LF), and edema factor (EF) (1,2 for review). PA binds to two related proteins on the cell surface; these are termed tumor epithelial marker 8 (TEM8)/anthrax toxin receptor (ATR) (3) and capillary morphogenesis protein 2 (CMG2) (4), although it is still unclear which is physiologically relevant. Following PA binding to its receptor, PA is cleaved into two fragments by a furin-like protease. The bound fragment binds both LF and EF; the resulting complex is then endocytosed which allows the translocation of LF and EF into the cytoplasm (5). EF is a calmodulin and Ca++-dependent adenylate cyclase responsible for the edema seen in the disease. It is thought to benefit the B. anthracis bacteria by inhibiting cells of the host immune system (reviewed in 6).