EPO and anemia (phenotype): EpoGFP/Δ3′E mice provide a handy and important source: lacking the 3′ enhancer demonstrated impaired hepatic Epo expression and profound anemia (Hct value was about 18% in newborn stage P4–6) and allowed us to directly sort REPs by FACS, which are constantly and stably labeled with GFP fluorescence in newborn kidneys.