the way function is achieved. They also consider that there are still more groups of connexin orthologs to be discovered. At gap junctions, the intercellular space is between 2 and 4 nm and hemichannels in the membrane of each cell are aligned with one another. Gap junction channels formed from two identical hemichannels are called homotypic, while those with differing hemichannels are heterotypic. In turn, hemichannels of uniform protein composition are called homomeric, while those with differing proteins are heteromeric. Channel composition influences the function of gap junction channels and different connexins will not necessarily form heterotypic with all others. Before innexins and connexins were well characterized, the genes coding for connexin gap junction channels were classified in one of three groups, based on gene mapping and sequence similarity: A, B and C (for example, , ). However, connexin genes do not code directly for the expression of gap junction channels; genes can produce only the proteins that make up gap junction channels. An alternative naming system based on this protein's molecular weight is the most widely used (for example: connexin43=GJA1, connexin30.3=GJB4). Levels of organization The hemichannel genes (DNA) are transcribed to RNA, which is then translated to produce the hemichannel proteins. One hemichannel protein has four transmembrane domains Six hemichannel proteins create one hemichannel. When different hemichannel proteins join to form one hemichannel, it is called a heteromeric hemichannel. Two hemichannels, joined across a cell membrane comprise a Gap Junction channel.When two identical hemichannel proteins come together to form a Gap junction channel, it is called a homotypic GJ channel. When one homomeric hemichannel and one heteromeric hemichannel come together, it is called a heterotypic gap junction channel. When two heteromeric hemichannels join, it is also called a heterotypic Gap Junction channel. Gap junction channels (tens to thousands) assemble within a macromolecular complex called a gap junction plaque. Properties of hemichannel pairs A hemichannel channel pair: Allows for direct electrical communication between cells, although different hemichannel subunits can impart different single channel conductances, from about 30 pS to 500 pS. Allows for chemical communication between cells, through the transmission of small second messengers, such as inositol triphosphate () and calcium (), although different hemichannel subunits can impart different selectivity for particular small molecules. In general, allows transmembrane movement of molecules smaller than 485 Daltons (1,100 Daltons through invertebrate gap junctions), although different hemichannel subunits may impart different pore sizes