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.
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.