Neurotoxin. Selectively blocks neuronal tetrodotoxin-sensitive voltage-gated sodium channels (Nav) with an IC(50) of 44.6 nM. Does not affect tetrodotoxin-resistant voltage-gated sodium channels or calcium channels. Monomer. Expressed by the venom gland. The presence of a 'disulfide through disulfide knot' structurally defines this protein as a knottin. LD(50) is 0.2 +-0.07 mg/kg by intraperitoneal injection into mice. Several genes are coding for Mu-theraphotoxin-Hhn1b for which the structure by NMR has been determined. The cross-references to PDB can be found in entry AC D2Y232. Belongs to the neurotoxin 10 (Hwtx-1) family. 22 (Htx-4) subfamily.