AID in Antibody Diversi fication: There and Back Again

Activation -Induced cytidine Deaminase (AID) initiates affinity maturation and isotype switching by deaminating deoxycytidines within immunoglobulin genes, leading to somatic hypermutation (SHM) and class switch recombination (CSR). AID thus potentiates the humoral response to clear pathogens. Marking the 20th anniversary of the discovery of AID, we review the current understanding of AID function. We discuss AID biochemistry and how error -free forms of DNA repair are co-opted to prioritize mutagenesis over accuracy during antibody di- versification. We discuss the regulation of DNA double -strand break (DSB) repair pathways during CSR. We describe genomic targeting of AID as a multilayered process involving chromatin architecture, cis- and trans -acting factors, and de- termining mutagenesis - distinct from AID occupancy at loci that are spared from mutation.