Charybdotoxin was originally isolated from the venom of the Israeli scorpion Leiurus quinquestriatus hebraeus. Charybdotoxin blocks KCa1.1 (large conductance Ca2+ activated K+, Slo) channels in nM concentrations2 as well as Kv1.2 (Kd, 14 nM) and Kv1.3 (Kd, 2.6 nM) channels. However, experiments with cloned KCa1.1 channels demonstrate the strong effect of the slobeta subunits on the potency of block by Charybdotoxin.