but unlike connexins do not appear to easily form gap junctions. Connexins have been found in only vertebrata and tunicata of over 20 different major metazoan groups. Innexins and pannexins are far more widespread including innexin homologues in vertebrates. Even the unicellular Trypanosomatidae parasites have unnexin genes presumably to aid in their infection of animals including humans. As we learn more the term "Gap Junction" appears to be the accurate description of a intercellular structure it was originally coined for more than a well defined type of intercellular communication. Interestingly gap junction like structures are found in sponges despite not having pannexins. While we are still at the early stages of understanding the nervous system of a sponge the "gap junctions" of sponges may as yet indicate intercellular communications pathways. Functions Gap junctions may be seen to function at the simplest level as a direct cell to cell pathway for electrical currents, small molecules and ions. The control of this communication allows complex downstream effects on multicellular organisms as described below. Embryonic, organ and tissue development In the 1980s, more subtle but no less important roles of gap junction communication have been investigated. It was discovered that gap junction communication could be disrupted by adding anti-connexin antibodies into embryonic cells. Embryos with areas of blocked gap junctions failed to develop normally. The mechanism by which antibodies blocked the gap junctions was unclear but systematic studies were undertaken to elucidate the mechanism. Refinement of these studies showed that gap junctions appeared to be key to development of cell polarity and the left/right symmetry/asymmetry in animals. While signaling that determines the position of body organs appears to rely on gap junctions so does the more fundamental differentiation of cells at later stages of embryonic development. Gap junctions were also found to be responsible for the transmission of signals required for drugs to have an effect and conversely some drugs were shown to block gap junction channels. Gap junctions, the "bystander effect" and therapies Cell death The "bystander effect" with its connotations of the innocent bystander being killed is also mediated by gap junctions. When cells are compromised due to disease or injury and start to die messages are transmitted to neighboring cells connected to the dying cell by gap junctions. This can cause the otherwise unaffected healthy bystander cells to also die. The bystander effect is, therefore, important to consider in diseased