An example of the generation of diverse protein functions through alternative splicing of genes important in neuronal functions: complex alternative splicing patterns of the middle exons of the mammalian BK potassium channel gene Slo (GenBank accession #: U23767). In A, the exon/intron organization of the Slo gene and alternative splicing positions, exon (boxes) sizes and possible number of variant transcripts produced from each position are indicated above the gene, with the exon numbers below it. The total number of channel subunits produced from all possible combinations of alternative splicing is 2×2×4×7×3×2=672, as indicated. Position of the STREX exon is indicated with an arrow. The length (nt) bar is for exon sizes. In B, positions where the alternative exons are inserted relative to the protein domains are indicated with arrows. S: transmembrane (or putative) domains. P: channel pore. Black oval: Ca++ bowl. The S4 voltage-sensitive domain is labelled with “+”s. (C) A channel heterotetramer of four different variant subunits a, b, c and d, with the total number of such heterotetramers indicated above the channel (see text for calculations). (D) Diagram of an action potential with depolarization, repolarization and afterhyperpolarization phases indicated. BK channels participate in the last two phases (arrows).