Clostridium botulinum and Clostridium tetani produce highly potent neurotoxins, called botulinum toxins and tetanus toxin, respectively. The clostridial neurotoxins specifically bind to neuronal cells and disrupt neurotransmisser release by cleaving proteins involved in specific vesicle membrane fusion. Each toxin is synthesized as an inactive approximately 150 kDa single-chain protein. The protein is posttranslationally proteolyzed to form the active dichain molecule in which the chains approximately 50 and approximately 100 kDa, remain linked by a disulfide bond. The structural organization is funcionally related to the fact that CNTs intoxicate neurons via four-step mechanism consisting of 1. binding, 2. internalization, 3. membrane translocation, and 4. enzymatic target modification. The L chain is responsible for the intracellular catalitic activity. The NH2-terminal 50-kDa domain of the H chain (HN) is implicated in membrane translocation, whereas the COOH-terminal part (HC) is mainly responsible for neurospecific binding.