The protease enzyme Lethal Factor (LF) is one of the three proteins (LF, EF & PA) composing the anthrax toxin produced by Bacillus anthracis, a bacteria which can infect many mammalian species and that may be fatal. LF is not toxic by itself, but when associated with Protective Antigen (PA), can then gain entry to cells. Once inside the cell, LF then cleaves the N terminal of most dual specificity mitogen activated protein kinase kinases (MAPKKs or MAP2Ks) (except for MAP2K5). Cleavage invariably occurs within the N terminal proline rich region preceding the kinase domain, thus disrupting a sequence involved in directing specific protein protein interactions necessary for the assembly of signaling complexes. There may be other cytosolic targets of LF involved in cytotoxicity. The proteasome may mediate a toxic process initiated by LF in the cell cytosol involving degradation of unidentified molecules that are essential for macrophage homeostasis. This is an early step in LF intoxication, but it is downstream of the cleavage by LF of MEK1 or other putative substrates.