Xiaosa Wu, Kanchan Gupta, and Kenton Swartz (left to right) reveal that members of the Kv1 channel family have different propensities to slow inactivate, but use the same mechanism of selectivity filter dilation. The T439V mutation in the Shaker channel greatly accelerates slow inactivation (A) whereas the equivalent mutation in Kv1.2, S371V, has less of an effect (B). This difference is erased, however, when S371V is combined with a second mutation in a critical residue near the selectivity filter, V381T (C).