The connexin family of proteins form hexameric complexes, called connexons, that facilitate movement of low molecular weight proteins between cells via gap junctions. Connexin proteins share a common topology of four transmembrane ?helical domains, two extracellular loops, a cytoplasmic loop and cytoplasmic N- and C-termini. Many of the key functional differences arise from specific amino acid substitutions in the most highly conserved domains; the transmembrane and extracellular regions. Connexin 31.9, also known as GJD3 (Gap junction delta-3 protein), is a 294 amino acid protein that interacts with ZO-1, a tight junction protein. Connexin 31.9 is expressed in heart, brain, kidney, spleen, lung, testis, colon and vascular smooth muscle cells. There are two isoforms of connexin 31.9 that are produced as a result of alternative splicing events.