Iron deficiency affects 30% of the world's population. Iron metabolism is tightly regulated, with both gut transport and storage being coordinated. Hereditary haemochromatosis due to mutations in the HFE gene leads to increased absorption of iron and multiple end-organ damage. Myelodysplastic disorders are acquired clonal stem-cell disorders that cause ineffective erythropoiesis. Aplastic anaemia is caused by an intrinsic defect of haemopoietic stem cells; both inherited and acquired forms occur. Primary polycythaemia is a myeloproliferative disorder, a non-malignant stem-cell disease.