 Intercellular gap junctions in vertebrates, including humans, are formed by the connexin family of proteins.Structurally, pannexins and connexins are very similar, consisting of 4 transmembrane domains, 2 extracellular and 1 intracellular loop, along with intracellular N- and C-terminal tails.Despite this shared topology, the protein families do not share enough sequence similarity to confidently infer common ancestry.