Uracil in DNA may result from incorporation of dUMP during replication and from spontaneous or enzymatic deamination of cytosine, resulting in U:A pairs or U:G mismatches, respectively. Uracil generated by activation-induced cytosine deaminase (AID) in B cells is a normal intermediate in adaptive immunity. Five mammalian uracil-DNA glycosylases have been identified; these are mitochondrial UNG1 and nuclear UNG2, both encoded by the UNG gene, and the nuclear proteins SMUG1, TDG and MBD4. Nuclear UNG2 is apparently the sole contributor to the post-replicative repair of U:A lesions and to the removal of uracil from U:G contexts in immunoglobulin genes as part of somatic hypermutation and class-switch recombination processes in adaptive immunity. All uracil-DNA glycosylases apparently contribute to U:G repair in other cells, but they are likely to have different relative significance in proliferating and non-proliferating cells, and in different phases of the cell cycle. There are also some indications that there may be species differences in the function of the uracil-DNA glycosylases.