Allegorical representation of the Shaker S4 helix (train riding a helical track) with its arginines (cars with white crosses) entering the omega pore (omega-shaped tunnel). Solutions ions (birds on the figure) cannot enter the pore when it is engaged by the arginines. When S4 is in its resting conformation only the first arginine is inside the "tunnel" and can occlude it. Substitution of that arginine with smaller non-charged amino acids allows solution cations to flow through the non-occluded omega pore. This figure was created by Camin Dean, based on a drawing from Francesco Tombola, and proposed for the cover of the Neuron issue containing the article: "Voltage-sensing arginines in a potassium channel permeate and occlude cation-selective pores" by F. Tombola, M.M. Pathak & E.Y. Isacoff (2005).