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Mu-conotoxins are a family of peptides from the venoms of predatory cone snails. Conotoxins, which are peptides consisting of 10 to 30 amino acid residues, typically have one or more disulfide bonds. Conotoxins have a variety of mechanisms of actions, most of which have not been determined. Mu-conotoxins have two types of cysteine arrangements, but the knottin scaffold is not observed.

Mu-conotoxins target the muscle-specific voltage-gated sodium channels, and are useful probes for investigating voltage-dependent sodium channels of excitable tissues. Mu-conotoxins target the voltage-gated sodium channels, preferentially those of skeletal muscle, and are useful probes for investigating voltage-dependent sodium channels of excitable tissues

Mu-conotoxin GIIIA (mu-CTX) is one of the potent peptide toxins of the conotoxin family, which preferentially blocks vertebrate skeletal muscle sodium channels. The toxicity of mu-CTX is almost comparable to that of tetrodotoxin. The peptide shares some of the same biochemical properties as other mu-conotoxins (the arrangement of cysteine residues and the conserved arginine that is thought to interact with residues near the channel pore), but it lacks hydroxyproline. In addition, mu-CTX is also a high-affinity ligand for the ecto-vestibule of selected isoforms of voltage-gated Na(+) channels, making it the first specific antagonist of voltage-gated sodium channels against tetrodotoxin discovered.

mu-Conotoxin (µ-CTX) is a receptor site I sodium channel blocker isolated from the sea snail Conus geographus that specifically inhibits Na+ flux in skeletal muscle and eel Na+ channels with high affinity by physically occluding the channel pore. µ-CTXs are peptides consisting of 22 amino acid residues with six cysteines that form three internal disulfide bonds, imparting extreme rigidity to the toxin molecule. The toxin contains a number of charged residues, including several positively-charged amino acids that have been shown to be critical for its biological activity. At physiological pH, the toxin carries a net charge of +6 or +7 respectively for the GIIIA and GIIIB subtypes. The three-dimensional structure of µ CTX resembles a tetragonal bipyramid with axial distances of approximately 25 and 20 Å.