Mice reconstituted with P210-transduced STAT5A-null BM cells have a significantly lower incidence of the pure CML-like disease. (A) The STAT5A genotype of B6;129S6 mice was determined by PCR, detecting the presence or absence of genomic sequences from the mouse STAT5A gene (290-bp fragment), and the neomycin (neo) phosphotransferase gene (480-bp) used in the original targeting construct.⁴⁰  STAT5A genomic and water (–) controls are shown at right. (B) Lethally irradiated STAT5A^+/+ mice were reconstituted with P210-transduced BM cells from 5-FU–primed STAT5A^–/– or STAT5A^+/+ donors and assessed for survival by the method of Kaplan and Meier. The number of mice (N) in each cohort is shown at right. The median survival of P210/STAT5A^–/–-reconstituted mice was 31 days, compared with 21 days for P210/STAT5A^+/+-reconstituted animals (P = .13). (C) Photomicrographs of blood smears (original magnification, × 40; scale bar = 50 μM) of representative animals with CML, CML/ALL mix, and ALL. Note that CML/ALL is comprised of a fairly equal distribution of granulocytes (single arrow) and lymphoblasts (double arrow). The respective MAC1/B220 FACS profiles are shown beneath each disease photomicrograph panel. Isotype control peaks are shown at left in gray, and quantitation of the number of MAC1 (myeloid)– or B220 (B-lymphoid)–positive cells is shown at the top of each panel. (D) Leukemia subtype of mice reconstituted with P210-transduced STAT5A^–/–, STAT5A^+/+, or STAT5A^+/– BM cells. NE indicates nonevaluable; other, macrophage-monocytic/basophil leukemia. The incidence of CML was significantly decreased in P210/STAT5A^–/–-reconstituted mice (P = .001 compared with STAT5A^+/+ mice; Fisher exact test).