Mu-conotoxins block voltage-gated sodium channels. Extremely potent inhibitor of Nav1.4/SCN4A (96% inhibition at 1 uM). The inhibition is very slowly reversible. Expressed by the venom duct. The cysteine framework is III (CC-C-C-CC). Classified in the M-5 branch, since 5 residues stand between the fourth and the fifth cysteine residues. Potent inhibitor of Nav1.2/SCN2A (IC(50)=34.1 nM) (PubMed:22773842). Also inhibits Nav1.3/SCN3A (10% inhibition at 75 nM), Nav1.4/SCN4A (45% inhibition at 75 nM), Nav1.6/SCN8A (45% inhibition at 75 nM), but does not inhibit Nav1.5/SCN5A and Nav1.8/SCN10A (75 nM tested) (PubMed:22773842). The inhibition is slowly reversible (PubMed:22773842). Belongs to the conotoxin M superfamily.