Mice lacking Stat5 die perinatally of severe anemia [6],[12],[33],[34],suggesting that the functions of Stat5 in erythropoiesis are essential to life.By contrast, EpoR-HM mice, which retain only the binary low-intensity p-Stat5signal, are viable and have near-normal basal erythropoiesis. This evidence concerns the gene STAT5B and anemia (phenotype).