Gap junctions are composed of transmembrane channels that link the cytoplasm of neighboring cells. They differ from other membrane channels since they exist between two cells. Gap junctions are relatively non-specific. They allow passive diffusion of small molecules up to 1000D. The junctions exist in almost all vertebrate and non-vertebrates cells. It is believed that gap junctions play an important for intercellular communications and affect growth and differentiation of cells. Gap junctional channel is composed of a hemichannel (connexon) in the cell membrane of one cell joined in mirror symmetry with a connexon in the opposing cell. Each connexon is an oligomer of six protein subunits that define the axial aqueous pore. Molecular cloning studies have identified a family of at least 12 highly related Connexins that are designated according to MW, Cx26-50. Hydropathy analyses of Cx sequences predicts 4 transmembrane (TM), 2 extracellular (EC) and 3 cytoplasmic (CP) domains. The EC, TM and N-terminal CP domains are well conserved among family members. The Central and C-terminal domains are highly variable in both sequence and size. The N- and C-termini are predicted to be cytoplasmic.