Chemotherapy-resistant blasts are characterized by high PIM1 expression. (A) In vivo chemotherapeutic (Chemo) treatment schedule. Four mice per group per PDX were treated via intraperitoneal injection and euthanized on day 5. Leukemic blasts (hCD45⁺ cells) were sorted from bone marrow, after which RNA was isolated. (B) RT-qPCR for CISH and PIM1 on residual blasts in the bone marrow after in vivo chemotherapy treatment. (C) RT-qPCR for CISH and PIM1 on bone marrow samples from nontreated (vehicle) mice, mice that were treated for 1 week with the chemotherapy schedule from panel A (chemo), and mice that relapsed after chemotherapy treatment (chemo relapse). *P < .05; **P < .01; ****P < .0001.