Cylindrospermopsin (abbreviated to CYN, or CYL) is a cyanotoxin produced by a variety of freshwater cyanobacteria. CYN is a polycyclic uracil derivative containing guanidino and sulfate groups. It is also zwitterionic, making it highly water soluble. CYN is toxic to liver and kidney tissue and is thought to inhibit protein synthesis and to covalently modify DNA and/or RNA.  It is not known whether cylindrospermopsin is a carcinogen, but it appears to have no tumour initiating activity in mice.
CYN was first discovered after an outbreak of a mystery disease on Palm Island, Queensland, Australia. The outbreak was traced back to a bloom of Cylindrospermopsis raciborskii in the local drinking water supply, and the toxin was subsequently identified. Analysis of the toxin led to a proposed chemical structure in 1992, which was revised after synthesis was achieved in 2000. Several analogues of CYN, both toxic and non-toxic, have been isolated or synthesised.
C. raciborskii has been observed mainly in tropical areas, however has also recently been discovered in temperate regions of Australia, North, South America, New Zealand and Europe. However, CYN-producing strain of C. raciborskii has not been identified in Europe, several other cyanobacteria species occurring across the continent are able to synthesize it.