Abstract: The present invention provides voltage-gated K+ channels with amino acid substitutions that produce a constitutively open phenotype when expressed in the cell. The invention includes mutated purified proteins with constitutively open voltage-gated K+ channel activity, for example those derived from Shaker, Shab, Shal, and Shaw family of voltage-gated K+ channels. The invention also includes nucleic acid encoding constitutively open voltage-gated K+ channels, as well as cells and transgenic animals expressing constitutively open voltage-gated K+ channels. Methods are provided for screening substances for ability to modulate voltage-gated K+ channels, by exploiting the ability of constitutively active voltage-gated K+ channels to affect growth or other phenotypic or genotypic characteristics in eukaryotic cells.