Aflatoxin B1 is the major analogue of a family of bisfuranocoumarin mycotoxins produced by Aspergillus flavus and related species. Aflatoxin B1 exhibits a distinctive UV spectrum and blue fluorescence. Aflatoxins are among the most potent mycotoxins known but are in fact "pre-toxins", requiring metabolic activation to the toxic principle. Aflatoxins are found widely in nature in trace amounts, particularly in grains and nuts. The toxicity of these metabolites was first recognised in the 1950s and their structures elucidated in 1963. Aflatoxins have been extensively reviewed.