T^HE voltage-activated K⁺, Na⁺ and Ca²⁺ channels are responsible for the generation and propagation of electrical signals in cell membranes. The K⁺ channels are multimeric membrane proteins formed by the aggregation of an unknown number of independent subunits^1–3. By studying the interaction of a scorpion toxin with coexpressed wild-type and toxin-insensitive mutant Shaker K⁺ channels, the subunit stoichiometry can be determined. The Shaker K⁺ channel is found to have a tetrameric structure. This is consistent with the sequence relationship between a K⁺ channel and each of the four internally homologous repeats of Na⁺ and Ca²⁺ channels^4–6.