Gap junctions are intercellular channels that link the cytoplasm of neighboring cells facilitating cell-cell communication. In non-chordates they are composed of innexins. Innexins are four transmembrane proteins that assemble into hexamers around a central channel. The association of hemichannels on adjacent membranes forms the gap junction which allows the direct passage of ions and small molecules. Gap junctions may be formed from homo- or heteromultimers. (Adapted from FBrf0219962).