AID and Apobec proteins have several, normally beneficial cellular functions. It has now been discovered that they act as DNA mutators in E. coli by deaminating cytosine in DNA. Thus, they may produce genome instability in mammals if not controlled.
Kunkel, T. A., & Diaz, M. (2002, November 1). Enzymatic cytosine deamination: Friend and foe. Molecular Cell. Cell Press. https://doi.org/10.1016/S1097-2765(02)00750-5