AICDA and myeloid sarcoma: Whether or not these cells initially trafficked into the CNS as naïve B cells, their coexpression of activation-induced cytidine deaminase (AID, an enzyme crucial for somatic hypermutation and class switch recombination of antibodies, that is normally expressed during activation of B cells in germinal centers) provides further support that the intrathecal milieu in patients with MS sustains accumulation of germinal center-like experienced B cells that can produce both IgM and IgG antibodies.