A steadily increasing number of cDNAs for proteins that are structurally related to the TRP ion channels have been cloned in recent years. All these proteins display a topology of six transmembrane segments that is shared with some voltage-gated channels and the cyclic-nucleotide-gated channels. The TRP channels can be divided, on the basis of their homology, into three TRP channel (TRPC) subfamilies: short (S), long (L) and osm (O). From the evidence available to date, this subdivision can also be made according to channel function. Thus, the STRPC family, which includes Drosophila TRP and TRPL and the mammalian homologues, TRPC1-7, is a family of Ca²⁺-permeable cation channels that are activated subsequent to receptor-mediated stimulation of different isoforms of phospholipase C. Members of the OTRPC family are Ca²⁺-permeable channels involved in pain transduction (vanilloid and vanilloid-like receptors), epithelial Ca²⁺ transport and, at least in Caenorhabditis elegans, in chemo-, mechano- and osmoregulation. The LTRPC family is less well characterized.