Pyrimidine dimers are molecular lesions formed from thymine or cytosine bases in DNA via photochemical reactions. Ultraviolet light (UV) induces the formation of covalent linkages between consecutive bases along the nucleotide chain in the vicinity of their carbon-carbon double bonds. Main pyrimidine dimer includes two kinds, and one kind is cyclobutane pyrimidine dimer (cyclobutane Pyrimidine dimers, CPDs), one kind is (6-4) photoproduct ((6-4) photoproducts, 6-4PPs).Its In, (6-4) photoproduct can be converted into its isomer Dewar formula photoproducts under the ultraviolet irradiation of lasting 313 nm (Dewar photoproducts, Dewar PP). In addition to pyrimidine dimer, in recent years, researcher also found that some are new Photoproduct, such as 5 methylcysteins or 5 methyl adenines.