Using expression of Ig isotypes in the BCR (IgD- and either IgM+, IgG3+, IgG1+ or IgA+) to define MBCs, we have demonstrated that CIS/MS patients exhibit three abnormalities that may be relevant to understanding how B cell dysfunction contributes to the immunopathogenesis of MS. This evidence concerns the gene CD40LG and myeloid sarcoma.