The Drosophila Shaker gene on the X chromosome has three sister genes, Shal, Shab, and Shaw, which map to the second and third chromosomes. This extended gene family encodes voltage-gated potassium channels with widely varying kinetics (rate of macroscopic current activation and inactivation) and voltage sensitivity of steady-state inactivation. The differences in the currents of the various gene products are greater than the differences produced by alternative splicing of the Shaker gene. In Drosophila, the transient (A current) subtype of the potassium channel (Shaker and Shal) and the delayed-rectifier subtype (Shab and Shaw) are encoded by homologous genes, and there is more than one gene for each subtype of channel. Homologs of Shaker, Shal, Shab, and Shaw are present in mammals; each Drosophila potassium-channel gene may be represented as a multigene subfamily in mammals.