in molluscs, arthropods and lancelets. Cis-defensins Arthropod defensins are the best-characterised defensins from invertebrates (especially those from insects). Other invertebrates known to produce defensins from this protein superfamily include molluscs, annelids and cnidaria. Plant defensins were discovered in 1990 and have subsequently been found in most plant tissues with antimicrobial activities, with both antifungal and antibacterial examples. They have been identified in all major groups of vascular plants, but not in ferns, mosses or algae. Fungal defensins were first identified in 2005. Studied examples mainly have anti-bacterial activities and have been found in both main divisions of fungi (Ascomycota and Basidiomycota), as well as in the more basal groups of Zygomycota and Glomeromycota. Bacterial defensins have also been identified, but are by far the least studied. They include variants with only four cysteines, whereas defensins from eukaryote defensins almost all have six or eight. Related defensin-like proteins In addition to the defensins involved in host defence, there are a number of related Defensin-Like Peptides (DLPs) that have evolved to have other activities. Toxins There appear to have been multiple evolutionary recruitments of defensins to be toxin proteins used in the venoms of animals; they act via a completely different mechanism to their antimicrobial relatives, from binding directly to ion channels to disrupting nerve signals. Examples include the crotamine toxin in snake venom, many scorpion toxins, some sea anemone toxins, and one of the toxins in platypus venom. Indeed, an insect defensin has been experimentally converted into a toxin by deletion of a small loop that otherwise sterically hindered interactions with the ion channels. Signalling In vertebrates, some α- and β-defensins are involved in signalling between the innate immune and adaptive immune systems. In plants, a specialised family of DLPs is involved in signalling to detect if self-pollination has occurred and induce self-incompatibility to prevent inbreeding. Enzyme inhibitors Some antimicrobial defensins also have enzyme inhibitory activity, and some DLPs function primarily as enzyme inhibitors, acting as antifeedants (discouraging animals from eating them). Function In immature marsupials, because their immune system is underdeveloped at the time of birth, defensins play a major role in defense against pathogens. They are produced in the milk of the mother as well as by the young marsupial in question. In human breast milk, defensins play a central role in neonate immunity. The human genome contains theta-defensin genes, but they have a premature stop codon, hampering their