Loss of Id1 accelerates the development of MLL-AF9–driven leukemia in the BM transplantation model. (A) The strategy of BM transplantation model. (B) In the primary transplantation assay, loss of Id1 shortens the survival time of recipient mice (46 days vs 74 days, n = 10 per group, P < .001). (C) In the secondary transplantation assay, the mice in Id1^−/− group developed leukemia faster compared with the wt group. (D) The morphology results show that, 6 weeks after transplantation, there were more leukemia blast cells in the PB, BM, and spleen of Id1^−/− group mice compared with the wt group. (E) The flow analysis shows that, 6 weeks after transplantation, the PB cells of the mice in Id1^−/− group express less GFP⁺c-Kit⁺ cells compared with the wt group. (F) The flow analysis of L-GMP (FcγR II/III^hiCD34⁺) (left panel) in the BM cells from wt and Id1^−/− MLL-AF9 leukemia mice. The frequency of L-GMP in the BM cells from Id1^−/− MLL-AF9 leukemia mice is higher than that from wt MLL-AF9 leukemia mice (right panel). (G) The mRNA expression level of Hoxa 9 in wt and Id1^−/− BM HSPCs (marked as wt and Id1^−/−), BM HSPCs transduced with MLL-AF9 (marked as MLL-AF9 preleukemia wt and Id1^−/−), and leukemia cells in resulting leukemia (marked as MLL-AF9 leukemia wt and Id1^−/−). (H) The columns represent the numbers of colonies in each plating of wt, Id1^−/−, or Id1^−/−p21^−/− BM cells transduced with MLL-AF9 (±SEM; n = 3). (I) The quantitative polymerase chain reaction (Q-PCR) analysis shows the Id1 expression in the MLL⁺ patient BM samples (age ≤3 group [n = 9] vs age >3 group [n = 8]). 5-FU, fluorouracil.