Binds specifically to voltage-gated sodium channels (Nav), thereby delaying their inactivation. This toxin has a great efficacy for Nav1.4/SCN4A (EC(50)=30.62 nM) and Nav1.5/SCN5A (EC(50)=62.5 nM). It is less potent on Nav1.6/SCN8A (EC(50)=about 300 nM), Nav1.1/SCN1A (EC(50)=390.55 nM), Nav1.3/SCN3A (EC(50)=459.36 nM) and Nav1.2/SCN2A (EC(50)=1998.00 nM) (when measured as the increase in the slow component). LD(50) is 450 ug/kg by intraperitoneal injection into mice. Belongs to the sea anemone sodium channel inhibitory toxin family. Type I subfamily.