For example, serum ferritin concentration is associated with iron reserves in bone marrow [4,7], soluble transferrin receptor (sTfR) is an indicator of the severity of iron insufficiency only when iron stores are depleted and no other causes of abnormal erythropoiesis are known [4], serum retinol concentration is associated with hepatic vitamin A concentration when the liver is depleted of vitamin A [8], and serum zinc concentration is associated with the rapidly exchangeable zinc pool size and clinical signs of zinc deficiency [6,9]. The gene discussed is TFRC; the disease is Zinc deficiency.