Blocks the Nav1.2/SCN2A, Nav1.4/SCN4A, Nav1.5/SCN5A and Nav1.6/SCN8A sodium channels. Shows a slight preference for the Nav1.2 and Nav1.4 channels. Reduces the peak amplitude of the sodium current and negatively shifts the steady-state inactivation process. Does not shift the threshold potential of activation or the voltage corresponding to maximal current. Does not change the reversal potential of the sodium current. May act on site 1 of the receptor. Expressed by the venom gland. The presence of a 'disulfide through disulfide knot' structurally defines this protein as a knottin. Contains 3 disulfide bonds. Does not inhibit the sodium channel Nav1.8/SCN10A. Belongs to the neurotoxin 19 (CSTX) family.