The percent conductance change is given as mean ± SD, with n being the number of single-channel reactions that were used in calculating each value. All results are from experiments with single-channel resolution. Low-pass filtering was 1,000 Hz, and the sample interval was 0.15 ms. The percent conductance change and state of reaction could not always both be determined for a given channel, so the number reacting in the open state plus that in the closed state does not necessarily add up to the n value given for the conductance change. All mutant channels were tested at positive voltage (30–120 mV) with solutions as in Fig. 2.