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. This evidence concerns the gene EPO and anemia.