The enzyme components of anthrax toxins EF (Edema Factor - Robertson et al. 1988) and LF (Lethal Factor - Bragg & Robertson 1989; Klimpel et al. 1994) bind to PA63:ANTXR1 (protective antigen, large fragment: Anthrax receptor 1) oligomers on the target cell surface. Binding of the two toxins to an oligomer is competitive and as many as four toxin molecules can bind to one oligomer (Elliott et al. 2000; Pimental et al. 2004).