Using retroviral gene transfer of a hyperactive mutant of Stat5 (caStat5a-S710F) into hematopoietic stem/progenitor cells and their transplantation into recipient mice, Moriggl and colleagues [29] demonstrated that a persistent activation of this signal transducer was sufficient to cause multi-lineage leukemia in mice. The gene discussed is STAT5B; the disease is leukemia.