 However, increasing evidence suggests that pannexins do not form gap junctions unless overexpressed in tissue and thus, differ functionally from innexins.Innexins have four transmembrane segments (TMSs) and, like the vertebrate connexin gap junction protein, innexin subunits together form a channel (an "innexon") in the plasma membrane of the cell.Two innexons in apposed plasma membranes can form a gap junction.