Gap junctions are usually found in clusters and enable intercellular communication by allowing the passage of small molecules between cells. They play important roles in different biological processes. These include differentiation, cell cycle synchronization, cellular development, neuronal activity and the immune response. Proteins involved in gap junction formation are composed of four transmembrane domains, two extracellular loops and one intracellular loop and intracellular N- and C-termini. Several consensus cysteine residues in the extracellular loop are essential and necessary for intercellular docking of gap junction hemichannels in the opposing cell membrane.