Activation-induced cytidine deaminase (AID), an enzyme involved in SHM and CSR during normal B cell differentiation [79], was found to be upregulated in UM CLL cells [80], and, even if expression could be restricted to a small fraction of the clone [6,81], AID seems to be functional with generation of isotype-switched transcripts and mutations in the pre-switch μ region [82,83]. Here, AICDA is linked to B-cell chronic lymphocytic leukemia.