Uracil (U) is one of the four nucleotide bases in RNA, with the other three being adenine (A), cytosine (C) and guanine (G). In RNA, uracil pairs with adenine. In a DNA molecule, the nucleotide thymine (T) is used in place of uracil. Uracil is a product of spontaneous deamination of cytosine and is potentially mutagenic. The monofunctional UNG DNA glycosylase is the main defense against uracil residues in DNA. It co-localizes with RPA and PCNA and is believed to remove uracil during replication. Uracil readily undergoes regular reactions including oxidation, nitration, and alkylation. While in the presence of phenol (PhOH) and sodium hypochlorite (NaOCl), uracil can be visualized in ultraviolet light. Uracil also has the capability to react with elemental halogens because of the presence of more than one strongly electron donating group.