During the stationary phase of the life cycle of Bacillus thuringiensis (Bt) is produced a parasporal body (crystal) containing insecticidal proteins called Cry toxins, which are used for development of transgenic crops and spray formulations for control of pest insects (Van Rie 2000). Although Bt has been used for two decades as a commercial biopesticide, the mode of action has only been described recently. Cry proteins are included into the pore-forming toxins (PFTs), one of the largest classes of bacterial toxins ). Many sequences of cry genes have been identified, including 56 families (Cry1-Cry56) and 180 subtypes (Cry1Aa, Cry4Ba, etc.). Of all of these proteins, some structures have been resolved by X-ray crystallography. It is interesting to note that the sequences are different but the structural topology is conserved, indicating a similar mechanism of action. All contain three structural domains: Domain I contains seven α-helixes and is involved in the oligomerization of the toxin and its insertion into the membrane, and Domains II and III are made up of β-sheets and are involved in the attachment to the receptors (Soberon et al. 2010).The mechanism of toxicity begins when the insect ingests food contaminated with spores; the crystal is then solubilized in the midgut, releasing protoxins which are activated by proteases, and the toxic fragment binds in sequential fashion to receptors located in the microvilli. The first receptor is a cadherin-like protein (CADP), whose interaction allows the oligomerization of the toxin. This oligomeric