Increased myeloid immaturity in STAT5A^–/– CML. Photomicrographs and FACS plots of representative mice with the CML-like illness induced by P210/STAT5A^–/– or P210/STAT5A^+/+ BM cells. Note the increased proportion of morphologically immature myeloid cells (arrows) in P210/STAT5A^–/– CML, associated with an increase in MAC1⁺ cells that lack expression of the mature granulocytic marker GR1. Cell percentage is depicted in the top right portions of each grid. Similar results were seen in 3 additional mice from each cohort (average values in “CML-related survival is prolonged in P210/STAT5A–/–-reconstituted mice.”