Id1 expression in the host BMM leads to AML acceleration. (A) The strategy of FLT or BMT. (B) Survival curves of Id1^+/+ and Id1^−/− recipients that underwent transplantation with AE9a-expressing fetal liver LSKs primarily (n ≥ 17; median survival, 101 vs 163 days). (C) Survival curves of Id1^+/+ and Id1^−/− recipients that underwent transplantation with MLL-AF9–expressing BM L-GMP cells primarily (n ≥ 19; median survival, 63 vs 77 days). (D-G) Representative flow cytometry profiles and quantification of the frequencies of the GFP⁺c-Kit⁺ leukemia blast cells in the BM and spleen of the indicated mice at 4 weeks after FLT or BMT (n = 10). (H-I) Representative flow cytometry profiles and quantification of the frequencies of the L-GMP cells in the BM of the indicated mice at 4 weeks after BMT (n = 10). (J) Colony-forming unit (CFU) assays analyzing the GFP⁺c-Kit⁺ leukemia blast cells sorted from FLT recipients BM or L-GMP cells sorted from BMT recipients BM. Average number of colonies generated from 1000 cells (n = 3). (K) Cell cycle analysis on AE9a-expressing LSKs and MLL-AF9–expressing L-GMP cells cocultured with Id1^+/+ BMSCs or Id1^−/− BMSCs for 5 days (n = 3). ∗P < .05; ∗∗P < .01; ∗∗∗P < .005; ∗∗∗∗P < .001.