Five of 14 different STAT5B mutations have been detected in ≥3 patients with T-ALL (Figure 1B), and these patients had almost twice as high white blood cell (WBC) counts as patients with unmutated STAT5B, suggesting that STAT5B hyperactivation correlates with higher leukemic burden and extramedullary cancer cell survival (Figure 1C). This evidence concerns the gene STAT5B and acute lymphoblastic leukemia.