A key player in vitamin D metabolism is the bone-derived hormone fibroblast growth factor 23 (FGF23), as it inhibits the enzyme 1-alpha hydroxylase and stimulates the enzyme 24-hydroxylase, resulting in the conversion of 25(OH)D into 24.25(OH)2D instead of into 1.25(OH)2D. We thus hypothesize that plasma FGF23 concentrations differ between MS patients and healthy controls. This evidence concerns the gene FGF23 and myeloid sarcoma.