A conserved family of genes related to Drosophila eag, which encodes a distinct type of voltage-activated K+ channel, has been identified. Three related genes were found in cDNA libraries from Drosophila, mouse, and human tissues. One gene is the mouse counterpart of eag; the other two represent additional subfamilies. The human gene maps to chromosome 7. Family members share at least 47% amino acid identity in their hydrophobic cores and all contain a segment homologous to a cyclic nucleotide-binding domain. Sequence comparisons indicate that members of this family are most closely related to vertebrate cyclic nucleotide-gated cation channels and plant inward-rectifying K+ channels. The existence of another family of K+ channel structural genes further extends the known diversity of K+ channels and has important implications for the structure, function, and evolution of the superfamily of voltage-sensitive ion channels. Voltage-activated ion channels are members of evolutionarily conserved multigene families. The Shaker (Sh) family of K+ channels comprises four related genes in Drosophila, each of which has one or more mammalian homologs. These genes define at least four subfamilies of voltage-activated K+ channels within the Sh family. Most of our present understanding of the structure and function of K+ channels is based on studies of the polypeptides encoded by these genes. Analysis of other K+ channels that are not members of the Sh family will expand our understanding of these channels. Genes encoding additional types of K+ channels can be identified in Drosophila via molecular analysis of mutations affecting membrane excitability. Mutations of eag, identified by their leg-shaking phenotype, cause repetitive firing and enhanced transmitter release in motor neurons, suggesting a possible defect in K+ channels. Molecular studies revealed that eag encodes a polypeptide structurally related both to K+ channels in the Sh family and to cyclic nucleotide-gated cation channels. Expression in Xenopus oocytes confirms that the eag polypeptide assembles into channels conducting a voltage-activated K+ -selective outward current. These results raise the question of whether eag is the prototype of a distinct K+ channel gene family analogous to the Sh family. Here we use low-stringency screens and degenerate PCRs to isolate relatives of eag from Drosophila, mouse, and human tissues. Sequence alignments indicate the existence of a conserved multigene family of eag-related loci in Drosophila and mammals. The eag family has important implications for understanding the structure, function, and evolution of voltage-activated K+ channels and cyclic nucleotide-gated cation channels.A conserved family of genes related to Drosophila eag, which encodes a distinct type of voltage-activated K+ channel, has been identified. Three related genes were found in cDNA libraries from Drosophila, mouse, and human tissues. One gene is the mouse counterpart of eag; the other two represent additional subfamilies. The human gene maps to chromosome 7. Family members share at least 47% amino acid identity in their hydrophobic cores and all contain a segment homologous to a cyclic nucleotide-binding domain. Sequence comparisons indicate that members of this family are most closely related to vertebrate cyclic nucleotide-gated cation channels and plant inward-rectifying K+ channels. The existence of another family of K+ channel structural genes further extends the known diversity of K+ channels and has important implications for the structure, function, and evolution of the superfamily of voltage-sensitive ion channels. Voltage-activated ion channels are members of evolutionarily conserved multigene families. The Shaker (Sh) family of K+ channels comprises four related genes in Drosophila, each of which has one or more mammalian homologs. These genes define at least four subfamilies of voltage-activated K+ channels within the Sh family. Most of our present understanding of the structure and function of K+ channels is based on studies of the polypeptides encoded by these genes. Analysis of other K+ channels that are not members of the Sh family will expand our understanding of these channels. Genes encoding additional types of K+ channels can be identified in Drosophila via molecular analysis of mutations affecting membrane excitability. Mutations of eag, identified by their leg-shaking phenotype, cause repetitive firing and enhanced transmitter release in motor neurons, suggesting a possible defect in K+ channels. Molecular studies revealed that eag encodes a polypeptide structurally related both to K+ channels in the Sh family and to cyclic nucleotide-gated cation channels. Expression in Xenopus oocytes confirms that the eag polypeptide assembles into channels conducting a voltage-activated K+ -selective outward current. These results raise the question of whether eag is the prototype of a distinct K+ channel gene family analogous to the Sh family. Here we use low-stringency screens and degenerate PCRs to isolate relatives of eag from Drosophila, mouse, and human tissues. Sequence alignments indicate the existence of a conserved multigene family of eag-related loci in Drosophila and mammals. The eag family has important implications for understanding the structure, function, and evolution of voltage-activated K+ channels and cyclic nucleotide-gated cation channels.