Ethidium bromide is an intercalating dye used as a nucleic acid stain in molecular biology. The main three-ring portion resembles a flat purine-pyrimidine base pair, and is able to slip into DNA between adjacent co-planar base pairs (intercalation). Under ultraviolet light, intercalated EtBr fluoresces with a strong pink-orange colour, and for this reason is used extensively as a dye in DNA experiments. Ethidium bromide is a very strong mutagen, carcinogen, and teratogen that causes mutations, cancer, and developmental defects, respectively.