(A) 50 AML patient samples were divided according to MFI ratio (anti-CXCR4 MFI/IgG MFI). Samples with higher MFI ratios (>5) had significantly higher let-7a expression than those with lower MFI ratios (<5) (P < 0.001; n = 25 per group). (B) 10 fresh AML samples were stained with CXCR4 antibody and sorted based on surface CXCR4 expression. The CXCR4^hi subpopulation showed substantially less let-7a. (C) Western blot of samples 1 and 2 showed that let-7a targets, such as ITGB3 and BCL-XL, were expressed much less in the CXCR4^lo subpopulation. Expression level relative to CXCR4^hi cells is shown below blots. (D) The CXCR4^lo subpopulation was more sensitive to Ara-C treatment. Specific apoptosis was calculated as (percent treatment apoptosis – percent spontaneous apoptosis)/(1 – percent spontaneous apoptosis) and expressed as a percentage. Results for samples 1, 7, 8, and 9 are shown. (E) Primary human AML cells were stably infected with let-7a or empty vector (EV) and expanded for xenograft models, which showed a 2.2-fold increase in let-7a versus control cells. (F) hCD45⁺ cells in mouse peripheral blood were determined using flow cytometry at different time points. hCD45⁺ cell percentage was significantly reduced with Ara-C treatment, which was more obvious in the let-7a–overexpressing group. (G) 3 representative mice per group were sacrificed on day 32. Spleens of the Ara-C–treated let-7a–overexpressing group were significantly smaller than the other groups, and immunohistochemical staining with anti-hCD45 antibody (original magnification, ×4) confirmed lower leukemia burden. Scale bar: 100 μm. (H) Median survival was significantly extended in the let-7a–overexpressing group compared with controls with Ara-C treatment. *P < 0.05, **P < 0.01.