Figure 1: Different repair systems for the principal types of DNA lesion produced by a wide range of factors. UV-light induces cyclobutane pyrimidine dimers or (6-4) photoproducts that are repaired by nucleotide excision repair and direct reversal systems. Alkylating agents can modify all of the bases and the phosphates of the DNA, and some repair proteins remove these alkyl adducts in a direct manner. Oxygen radicals modify DNA, and the base excision repair system acts to reverse these changes. The main cause of spontaneous mutation is deamination, and base excision repair and alternative repair systems remove the lesions. Other bulky adducts or interstrand cross-links are repaired by the nucleotide excision repair system. The mismatch repair pathway repairs replication errors. Double-strand breaks and four-way junctions are induced by X-rays and are repaired by recombinational repair.