Further imaging and laboratory studies eventually supported the diagnosis of MS.<h4>Discussion</h4>While the comorbid association of MS and cblC in our patient may remain anecdotic, we suggest measuring Hcy and MMA levels in young patients with a relapsing-remitting demyelinating disorder, in order not to miss a cblC defect, that requires a specific and effective treatment. The gene discussed is CBLC; the disease is myeloid sarcoma.