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One of these is I AICDA i , encoding activation-induced cytidine deaminase (AID), which is highly expressed by GC B cells in vivo, and is induced in B cells in vitro in response to CD40L and cytokines such as IL-4 or IL-21 [[3], [5]]. After these initial events, some responding B cells will also form germinal centers (GC) where somatic hypermutation (SHM) of Ig variable region genes expressed by Ag-specific B cells occurs. Despite these similarities, there are key differences in function of these mutant AID proteins, as heterozygous-truncated AID-exon 5 protein maintained peripheral B cell tolerance, but the AR I c.428-1G i > I T AICDA i variant did not. Interestingly, ~ 25% of individuals with AR AID deficiency also develop IgM-mediated autoimmune phenomena (cytopenia, AIHA), revealing a key role for AID in maintaining peripheral B cell tolerance [[9]]. [Extracted from the article]