Despite expression of hyperactive Stat5 in over 90% of erythroid and B cells as well as the majority of macrophages, MMTV-tTA TetO-Stat5 transgenic mice did not develop multi-lineage leukemia similar to the retroviral gene transfer and transplantation model generated by Moriggl and colleagues [29]. The gene discussed is STAT5B; the disease is leukemia.