,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 The gene discussed is STAT5A; the disease is anemia (phenotype).