Gap junctions are clusters of intercellular channels connecting adjacent cells and permitting the direct exchange of ions and small molecules between cells. These channels are composed of two hemichannels, or connexons, one located on each of the two neighboring cells. Each connexon is composed of 6 trans-membrane protein subunits of the connexin (Cx) family. A gap of approximately 3 nm remains between the adjacent cell membranes, but two connexons interact and dock head-to-head in the extra-cellular space forming a tightly sealed, double-membrane intercellular channel (see Segretain and Falk, 2004). The activity of these intercellular channels is regulated, particularly by intramolecular modifications such as phosphorylation which appears to regulate connexin turnover, gap junction assembly and the opening and closure (gating) of gap junction channels.