Connexin proteins form gap-junctions between cells. They carry four transmembrane regions, hence why this family now includes Connexin_CCC, which represented the second pair of TMs.

1: Sequence and phylogenetic analyses of 4 TMS junctional proteins of animals: connexins, innexins, claudins and occludins. Hua VB, Chang AB, Tchieu JH, Kumar NM, Nielsen PA, Saier MH Jr; J Membr Biol. 2003;194:59-76. PMID:14502443