Exploratory analysis of the contribution of IgA IF to κ-FLC index revealed qualitatively the same results.<h4>Conclusion</h4>Increase of κ-FLC index in patients with MS is predominantly due to an intrathecal IgG synthesis, while the contribution of intrathecal IgM is less frequent and quantitatively low. This evidence concerns the gene CD79A and myeloid sarcoma.