GSTM1 and anemia (phenotype): Whilst the RBC number and hemoglobin concentration (Hb) define blood O2-carrying capacity, homeostatic modulation of Hb–O2 affinity ultimately regulates O2 capture/release in a manner that stabilizes O2 delivery in the setting of reduced O2 availability (i.e., hypoxia and anemia) or increased consumption (i.e., stress and disease).