EPO and anemia (phenotype): Because of the association of anemia with high serum EPO in extremely premature infants and because of the known increase in serum Epo in the hWtEPOR mice,26 we measured hematocrits in WT and hWtEPOR mice raised in RA or the OIR model to assess whether hWtEPOR mice experienced anemia (Figs. 2A, 2B).