Activation-induced (cytidine) Deaminase

Activation-induced (cytidine) deaminase (AID) is a 24 kDa enzyme that creates mutations in DNA.

AID removes the amino group from a cytidine base, turning it into a uridine (which is recognized as a thymidine). In other words, it changes a CG base pair into a UG base pair, which the cell's DNA repair machinery recognizes as a TG base pair, and finally ends up as a TA base pair.

In B cells in the lymph nodes, AID causes mutations that produce antibody diversity, but that same mutation process leads to B-cell lymphoma.

AID is currently thought to be the master regulator of secondary antibody diversification. It is involved in the initiation of three separate immunoglobulin (Ig) diversification processes: somatic hypermutation (SHM), in which the antibody genes are shuffled to provide enough diversity to deal with millions of antigens; class switch recombination (CSR), in which B cells change their expression from IgM to IgG or other immune types; and gene conversion (GC).

AID has been shown in vitro to be active on single stranded DNA, and has been shown to require active transcription in order to exert its deaminating activity. The involvement of Cis-regulatory factors is suspected as AID activity is several orders of magnitude higher in the immunoglobulin "variable" region than other regions of the genome that are known to be subject to AID activity. This is also true of artificial reporter constructs and transgenes that have been integrated into the genome.