Some mutagens are analogs of the usual bases that have ambiguous pairing properties; their mutagenic action results from their incorporation into DNA in place of one of the regular bases. Figure 4.20 show the example of bromouracil (BrdU), which is an analog of thymine that contain a bromine atom in place of the methyl group of thymine. BrdU is incorporated into DNA in place of thymine. But it has ambiguous pairing properties, because the presence of the bromine atom allows a shift to occur in which the base changes structure From a keto (=O) from to an enol (-OH) from. The enol from can base pair with guanine, which leads to substitution of the original A ‘T pair by a G’ C pair.e..