Conditional deletion of Id1 inhibits leukemia progression in vivo. (A) Conditional deletion of Id1 by tamoxifen treatment significantly prolongs the survival time of recipient mice transplanted with AE9aId1fl/flCreER cells compared with the vehicle-treated (corn oil) group (51 days vs 37 days, n = 21, P < .01). (B) The expression of Id1 is minimal in the sorted GFP⁺ cells isolated from the bone marrow of the mice in the tamoxifen treatment group described in panel A. (C) In vivo luciferase imaging shows that deletion of Id1 by tamoxifen impairs leukemia progression in recipient mice transplanted with AE9aId1fl/flCreER/MSCV-luciferase cells (compared with the corn oil control group). (D) Conditional deletion of Id1 significantly decreases the frequencies of C-Kit⁺GFP⁺ leukemia blast cells and GFP⁺Gr1⁺ leukemia cells and increases the frequencies of normal GFP⁻CD45/CD45.2⁺ cells in the peripheral blood of mice transplanted with AE9aId1fl/flCreER cells. (E) Survival of mice receiving different dilutions of AE9a-transduced Id1fl/fl or Id1Δ/Δ fetal liver cells (n = 5 per group, left panel). Conditional deletion of Id1 significantly decreases the frequency of leukemia-initiating cells, in the limiting dilution assay (right panel).