The voltage dependence of K, Na, and Ca2 channels is brought about by a voltage sensor that moves 12–13 e0 across the entire electric field (Schoppa et al., 1992; Hirschberg et al., 1996; Noceti et al., 1996). In the case of Shaker K channel it is known which residues are responsible for this large amount of gating charge. This was found by measuring the total gating charge movement per channel after each of the putative charged residues (basic or acidic) were neutralized one by one (Aggarwal and MacKinnon, 1996; Seoh et al., 1996). These studies revealed that the four most extracellularly located basic residues of the S4 segment (R362, R365, R368, and R371) and the most intracellular acidic residue in the S2 segment (E293) are the major contributors to the gating charge movement. The simple assumption that all these residues move across the entire electric field accounts for more than the measured total gating charge. This means that the movement of at least some of the charged residues is only partial within the electric field.