Iron is stored as iron(III) in ferritin. The exact nature of the binding site has not yet been determined. The iron appears to be present as a hydrolysis product such as FeO(OH). Iron is transported by transferrin whose binding site consists of two tyrosines, one aspartic acid and one histidine. The human body has no mechanism for iron excretion. This can lead to iron overload problems in patients treated with blood transfusions, as, for instance, with β-thalassemia.  Iron is actually excreted in urine and is also concentrated in bile which is excreted in feces.