RUNX1 inhibits AML caused by MLL fusion proteins. (A) The immunoblot of RUNX1 and CBFβ in the MV4-11 cell line infected with recombinant amphotropic retroviruses carrying either pMY blank vector alone or with RUNX1. eGFP⁺ cells were sorted after infection and used for western blotting. (B) Growth curves of cells obtained in panel A. (C) Morphology of cells obtained in panel A by cytospin. BM cells from preleukemia (D-F) or leukemia (G-I) staged Mll-Af9 knock-in mice were infected with retroviruses carrying either pMY blank vector alone or with pMY-RUNX1. The eGFP⁺ cells were used for CFU replating assays or transplanted into lethally irradiated mouse recipients. Colony formation of each plating (E,H) and the representative image of colonies (D,G, upper panels) and cytospin (D,G, lower panels) of the first round of plating are shown (**P < .01; ***P < .001). (F,I) The survival curve of transplanted mice. (J-L) Low-density BM cells from Runx1^flox/wtCbfβ^flox/wt/Rosa-Cre-ERT2 mice were infected with retroviruses carrying MLL-AF9, and eGFP⁺ cells were sorted for CFU plating assays with or without tamoxifen (4-OHT) (J-K) or for BMT (L). Arrows in panel L indicate the time points when 4-OHT or corn oil was injected into the treatment group or control group separately. (M) Endogenous Runx1 and Cbfβ proteins were measured by immunoblotting with leukemic cells in panel J. ns, not significant.