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A connexin is one of 20 odd gap junction proteins that can be components of a hexamer connexon making up a gap junction through which low molecular weight materials diffuse from one cell to a neighbouring cell. The connexin structure is of four membrane-spanning domains, two extracellular loops, an intracellular loop and flanking N- and C-termini also found intracellularly.

Connexins are assembled on the membrane of the endoplasmic reticulum and migrate to the cell membrane where they then match with connexons of an adjacent cell. Some connexins can form unapposed hemichannels (eg. gap junction alpha-1, beta-1, beta-2 and delta-2 proteins) and when they do this may have similar roles to another class of similar protein called the pannexins whose function is less well characterised. In invertebrates innexins seem to do the same as connexins and pannexins. The connexin gene family is believed to have 21 members in the sequenced human genome: