VDR and myeloid sarcoma: Both forms are converted to 25-hydroxyvitamin D and stored in the liver prior to being activated in the kidney to 1,25-hydroxyvitamin D. Multiple clinical studies have found a correlation between vitamin D levels and the progression of MS, which may be attributable to the essential immunomodulatory function of vitamin D. Because most immune cells express the vitamin D receptor and 1-hydroxylase, vitamin D influences the phagocytic activity of macrophages and natural killer (NK) cells.