Stat5^null/+ cells depending on p210^BCR-ABL1 or v-ABL are highly susceptible to imatinib-induced apoptosis. (A) BM derived from Stat5^null/+ (n = 4) and Stat5^+/+ (n = 6) mice were infected with a retrovirus encoding for p210^BCR-ABL1-IRES-GFP and subsequently treated for 48 hours with increasing dosages of imatinib as indicated. Percentages of BCR-ABL1⁺/GFP⁺ cells were analyzed via FACS to evaluate differences in imatinib response. Stat5^null/+ cells reacted significantly more sensitive on imatinib treatment compared with Stat5^+/+ cells. Shown is one representative FACS profile. (B) Summary of all experiments described in panel A. (C) Immunoblot analysis of STAT5A/B and v-ABL protein expression in Stat5^null/+ and Stat5^+/+ cells. (D) Dose-response curves determined by [³H]-thymidine incorporation of v-ABL transformed Stat5^null/+ and Stat5^+/+ cells treated with imatinib for 24 hours (n = 4/genotype). (E) Summary of colony-forming assays of v-Abl⁺Stat5^null/+ and Stat5^+/+ cells grown for 21 days in growth-factor free methylcellulose enriched with 10nM or 100nM imatinib (n = 3/genotype). (F-G) Annexin V stained v-ABL transformed Stat5^null/+ and Stat5^+/+ cells challenged with 100nM imatinib for 24 hours (right, n = 3/genotype). One representative FACS profile is shown in panel G. Data are mean ± SD. *P < .05. **P < .001.