The members of this family resemble neurotoxin B-IV , which is a crustacean-selective neurotoxin produced by the marine worm Cerebratulus lacteus. This highly cationic peptide is approximately 55 residues and is arranged to form two antiparallel helices connected by a well-defined loop in a hairpin structure. The branches of the hairpin are linked by four disulphide bonds. Three residues identified as being important for activity, namely Arg-17, -25 and -34, are found on the same face of the molecule, while another residue important for activity, Trp30, is on the opposite side. The protein's mode of action is not entirely understood, but it may act on voltage-gated sodium channels, possibly by binding to an as yet uncharacterised site on these proteins. Its site of interaction may also be less specific, for example it may interact with negatively charged membrane lipids .