,11 The STAT5 is an essential downstream mediator of cytokine signaling at multiple stages of hematopoiesis.12, 13, 14, 15, 16 In eutherian mammals, 2 closely related STAT5 isoforms,17 STAT5A and STAT5B, display distinct and redundant functions in different cell types.18, 19, 20, 21 Mice that lack both genes, or the N-terminal domains of both genes, develop severe anemia and leukopenia22, 23, 24, 25, 26 that are associated with reduced survival and proliferation of erythroblasts.15 This evidence concerns the gene STAT5B and anemia.