Gap junctions are found at points of cell-cell contact where they form an aqueous pore through which small hydrophilic molecules and ionic charge may pass. Each gap junction comprises two hemichannels, or connexons that are composed of six connexin (Cx) subunits. Although each connexon may be composed of a mix of connexin subtypes, Cx37, Cx40 and Cx43 are particularly associated with mammalian endothelium and vascular smooth muscle.