and different charge selectivity. Large biomolecules, for example, nucleic acid and protein, are precluded from cytoplasmic transfer between cells through gap junction hemichannel pairs. Ensures that molecules and current passing through the gap junction do not leak into the intercellular space. To date, five different functions have been ascribed to gap junction protein: Electrical and metabolic coupling between cells Electrical and metabolic exchange through hemichannels Tumor suppressor genes (Cx43, Cx32 and Cx36) Adhesive function independent of conductive gap junction channel (neural migration in neocortex) Role of carboxyl-terminal in signaling cytoplasmic pathways (Cx43) Occurrence and distribution Gap Junctions have been observed in various animal organs and tissues where cells contact each other. From the 1950s to 1970s they were detected in crayfish nerves, rat pancreas, liver, adrenal cortex, epididymis, duodenum, muscle, Daphnia hepatic caecum, Hydra muscle, monkey retina, rabbit cornea, fish blastoderm, frog embryos, rabbit ovary, re-aggregating cells, cockroach hemocyte capsules, rabbit skin, chick embryos, human islet of Langerhans, goldfish and hamster pressure sensing acoustico-vestibular receptors, lamprey and tunicate heart, rat seminiferous tubules, myometrium, eye lens and cephalopod digestive epithelium. Since the 1970s gap junctions have continued to be found in nearly all animal cells that touch each other. By the 1990s new technology such as confocal microscopy allowed more rapid survey of large areas of tissue. Since the 1970s even tissues that were traditionally considered to possibly have isolated cells such as bone showed that the cells were still connected with gap junctions, however tenuously. Gap junctions appear to be in all animal organs and tissues and it will be interesting to find exceptions to this other than cells not normally in contact with neighboring cells. Adult skeletal muscle is a possible exception. It may be argued that if present in skeletal muscle, gap junctions might propagate contractions in an arbitrary way among cells making up the muscle. At least in some cases this may not be the case as shown in other muscle types that do have gap junctions. An indication of what results from reduction or absence of gap junctions may be found by analysis of cancers or the aging process. Since the discovery of innexins, pannexins and unnexins, gaps in our knowledge of intercellular communication are becoming more defined. Innexins look and behave similarly to connexins and so can be seen to fill a similar role to connexins in invertebrates. Pannexins also look individually similar to connexins