Immunodeficiency with hyper IgM type 5 is caused by a mutation in the Uracil-DNA glycosylase (UNG) gene, which, like AICDA, is located on chromosome 12.
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They are similar enough to be incorporated as part of DNA replication, but they possess side groups on the uracil component (an iodine and a CF3 group, respectively), that prevent base pairing.
Four different uracil-DNA glycosylase activities have been identified in mammalian cells, including UNG, SMUG1, TDG, and MBD4.
Uridine phosphorylase adds ribose-1-phosphate to the free base uracil, forming uridine monophosphate.