Gap junctions are the main site of cell-cell signaling or communication that allow small molecules to diffuse between adjacent cells. In vertebrates, gap junctions are composed of transmembrane proteins called connexins.  They form hexagonal pores or channels through which ions, sugars, and other small molecules can pass.  Each pore is made of 12 connexin molecules; 6 form a hemichannel on one cell membrane and interact with a hemichannel on an adjacent cell membrane. The permeability of these junctions is regulated by many factors including pH and Ca concentration.