Activation-induced cytidine deaminase (AID) catalyzes the deamination of cytosine into uracil, creating DNA mutations, and AID is involved in immunoglobulin class-switch recombination (CSR) and somatic hypermutation.162 Biallelic mutations in AICDA result in AID deficiency, which is the most common B-cell-intrinsic CSR defect.163,164 Autosomal-recessive AID deficiency causes hyper-IgM syndrome with low IgG and IgA levels. The gene discussed is AICDA; the disease is hyperinsulinemic hypoglycemia, familial, 4.