AICDA encodes activation-induced cytidine deaminase (AID), a single-stranded DNA-specific enzyme critical for adaptive immunity. AID catalyzes cytidine-to-uridine deamination within immunoglobulin variable and switch regions, initiating somatic hypermutation (SHM) and class-switch recombination (CSR)—two essential processes for generating high-affinity, isotype-diversified antibodies 1. During transcription of immunoglobulin loci, AID converts cytosine to uracil; the resulting deoxyuracil lesions are processed through error-prone DNA repair pathways to generate point mutations and isotype switching 2. AID expression is tightly regulated in germinal center B cells through CD40 signaling and transcriptional factors including Pax5 and Irf8, while posttranslational modifications regulate AID activity and nuclear-cytoplasmic localization 3. Transcription-coupled AID activity requires ELOF1-stabilized RNA polymerase II pausing to provide a platform for both damage induction and repair 4. Biallelic AICDA mutations cause autosomal recessive hyper-IgM syndrome type 2 (HIGM2), characterized by impaired CSR and SHM, while heterozygous nuclear export signal domain variants cause autosomal dominant HIGM2 with variable SHM defects 5. Beyond antibody diversification, AID may participate in DNA demethylation and epigenetic regulation 6.