Gap junctions are channel-forming structures that allow direct metabolic and electrical communication between adjacent cells of almost all types in mammalian tissues. In the human body, they are absent only in adult skeletal muscle cells and some circulating blood cells. A gap junction is formed two hemichannels, one in each of the neighboring cells, composed of six subunits. In mice and humans, at least 20 connexin and 3 pannexin genes encode gap junction proteins. Connexins are only found in chordates, while pannexins are present in both chordate and invertebrate genomes. Pannexins, previously known as innexins, are predicted to have four transmembrane regions, two extracellular loops, one intracellular loop, and intracellular N- and C-termini. Both human and mouse genomes contain three pannexin-encoded genes. Pannexin 2 (Px2, PANX2) appears to be a brainspecific gene, and is abundantly expressed in the central nervous system, as is pannexin 1. In many neuronal cell populations, including hippocampus, olfactory bulb, cortex, and cerebellum, pannexin 1 and pannexin 2 are co-expressed; in other brain regions such as white matter, only pannexin 1-positive cells are found.