A very low prevalence of CSF-restricted OCB has also been found in other diseases that were previously considered ‘variants’ of MS and are now recognised as immunologically distinct disease entities in their own rights by most experts, namely in AQP4-IgG-positive NMO [3, 4] and in MOG-EM [11] (Fig. 1). Here, AQP4 is linked to myeloid sarcoma.