Inclusion criteria are adult patients with stage 4 or 5 chronic kidney disease (including dialysis patients) with significant anaemia (haemoglobin ≤ 110 g/L) for at least 3 months for which there is no clear identifiable cause and that is unresponsive to large doses of either erythropoietin (≥ 200 IU/kg/week) or darbepoetin (≥ 1 μg/kg/week). This evidence concerns the gene EPO and anemia.