Leukemic blasts can be cultured ex vivo and retransplanted and are susceptible to direct Csnk1a1 and CDK7/9 inhibition. (A) Scheme showing the potential applications of leukemic blasts: Csnk^–/+p53 were expanded in vitro, transplanted into sublethally irradiated recipients, and used for small-scale drug screening based on viability. (B) In vitro expanded Csnk1a1^–/+ p5^mut blasts retain morphology of initial disease. Scale bar, 50 μm. (C) Platelet and white blood cell counts of sublethally irradiated (6 Gy) WT recipients of leukemic p53 blasts expanded in vitro (n = 3 mice) compared with controls with transplanted secondary leukemias. (D) Blast cells in the bone marrow were detected in 2 out of 3 mice that received transplantation with leukemic p53 blasts and compared to secondary leukemia transplants. (E) Bone marrow cellularity in mice that received transplantation with leukemic p53 blasts expanded in vitro. (F) HE staining of bone marrow (femur) section of mice transplanted with leukemic p53 blasts expanded in vitro. (G) Cell proliferation and survival (percentage viability) measured by MTT assay of cultured blasts (Csnk1a1–^/+p53), Csnk1a1^–/+ Hoxb8-Flt3, and WT Hoxb8-Flt3 cells subjected to increasing doses of D4476, A51, Alisertib, and Nutlin3a.