Gap junctions are regions of contact between adjacent cells, consisting of arrays of channels linking the cell interiors. The channels are formed by polypeptides called connexins; the amino acid sequences of many different connexins are known, and they are thought to resemble each other closely in tertiary and quarternary structure. Single channels have recently been isolated and purified, and earlier evidence has been confirmed showing that they consist of six identical subunits arranged around the central pore. Gap junction channels are known to open and close in response to changes in ligand concentrations and electrical potential; in this respect they are very similar to ligand-gated ion channels which act as receptors in the membranes of excitable cells. The similarity is shown to extend to structural features such as the amino acid residues lining the pore, and perhaps the location of the actual gate.