The low oxygen carrying capacity of the blood in anemia is compensated by an increased concentration of 2,3-Bisphosphoglycerate (2,3-BPG, alias 2,3-DPG), an allosteric effector promoting the offloading of oxygen to the tissues by lowering Hb-O2 affinity3. The gene discussed is GSTM1; the disease is anemia (phenotype).