Eukaryotic cells have multiple mechanisms for repairing damaged DNA. O6-methylguanine-DNA methyltransferase directly reverses some simple alkylation adducts. However, most repair strategies excise lesions from DNA. Two major pathways are base excision repair (BER), which eliminates single damaged-base residues, and nucleotide excision repair (NER), which excises damage within oligomers that are 25-32 nucleotides long. The specialized DNA glycosylases and AP endonucleases of BER act on spontaneous and induced DNA alterations caused by hydrolysis, oxygen free radicals, and simple alkylating agents. NER utilizes many proteins (including the XP proteins in humans) to remove the major UV-induced photoproducts from DNA, as well as other types of modified nucleotides. Different DNA polymerases and ligases are used to complete the separate pathways. Some organisms have alternative schemes, which include the use of photolyases and a specific UV-endonuclease for repairing UV damage to DNA. Finally, double-strand breaks in DNA are repaired by mechanisms that involve recombination proteins and, in mammalian cells, a DNA protein kinase.