Among various vertebrate species, three genes are known to encode two-pore segment channels (TPCs) termed TPC1-3. Interestingly TPC3 seems to be absent from the genomes of primates and rodents. The primary sequence of these channels indicates the presence of two putative pore-forming repeats. Each repeat contains six transmembrane domains and a pore loop, a structure strikingly reminiscent of many voltage-gated Na+ (NaV) and Ca2+ (CaV) channels. These twelve transmembrane structures are further thought to form functional dimers.