Treatment of leukemic mice with imatinib selects for STAT5A overexpressing cells. (A) Top panel: FACS analysis of CD19 and GFP from PB-derived cells of a healthy NOD/SCID mouse (left panel), freshly established stable v-Abl⁺ cells infected with pMSCv-eGFP based retrovirus encoding for murine STAT5A showing 1% infection efficiency (middle panel) and blood-derived cells from a NOD/SCID mouse 13 days after injection of the v-Abl⁺ cells (right panel). (B) FACS analysis of PB, lymph nodes (LN), spleen (SP), and BM after imatinib treatment by gavage (100 mg/kg per day). Percentage of CD19⁺/GFP⁻ (blue) and CD19⁺/GFP⁺ (red) cells is shown. (C) Summary of the in vivo experiment shown in panels A and B for a total of 6 mice. Error bars represent mean ± SD. (D) Representative FACS profile of v-Abl⁺ cells before injection (top panel), 13 days after injection (middle panel), and 9 days after in vivo imatinib treatment (bottom panel). Fold difference of GFP-mean fluorescence intensity of GFP⁺ (S5a^high) to GFP⁻ (S5a^low) cells are indicated. (E) pSTAT5 level of ex vivo derived S5a^high cells 9 days after in vivo imatinib (100 mg/kg per day) treatment (bottom histograms) and in vitro control S5a^high cells before and after 24 hours after imatinib (1000nM) addition (top histograms).