Aflatoxin B1 and B2 are among the main types of aflatoxins. Aflatoxin B1 is the most abundant, toxic and carcinogenic of the Aflatoxins. Aflatoxins have closely similar chemical structures, as all contain a bifuran ring fused to the coumarin nucleus. The B-group aflatoxins have a cyclopentanone ring bound to the coumarin structure. The B designation of aflatoxins B1 and B2 results from the emission of blue fluorescence when they absorb ultraviolet light.