Initiation of Notch1-driven T-ALL is Tcf7-dependent. (A) Schematic representation of BM chimeras transplanted with the indicated genotypes: R26 N1IC^lox/+ or R26 N1IC^lox/+ Tcf7^lox/lox (Controls, gray), R26 N1IC^lox/+Mx1Cre (N1IC, red), or R26 N1IC^lox/+ Tcf7^{lox/loxMx1Cre} (N1IC Tcf7^lox/lox, blue) and treatment schedule. (B) Kaplan-Meier survival plot of chimeras after last poly(I:C) injection. N1IC mice (n = 8), N1IC Tcf7^Δ/Δ (n = 10), and Controls (n = 7) followed for 199 days. Log-rank (Mantel-Cox) test, ****P value < .0001. (C-D) Phenotypic flow cytometric analysis of transplanted (CD45.2⁺) and induced (eGFP⁺ for N1IC and N1IC Tcf7^Δ/Δ) BM cells from Controls (n = 4), N1IC (n = 6), and N1IC Tcf7^Δ/Δ (n = 5). Quantification of absolute numbers is shown for T cells, erythroid cells, myeloid cells, and B cells. Data are represented as mean ± standard error of the mean (SEM). Unpaired t-test, *P value < .05; **P value < .01; ***P value < .001; ****P value < .0001. (E) Schematic representation of BM chimeras with Controls (gray, n = 3), R26 N1IC^{lox/lox Mx1Cre} (N1IC^Δ/Δ, red, n = 18), R26 N1IC^lox/lox Tcf7^{lox/lox Mx1Cre} (N1IC^Δ/Δ Tcf7^Δ/Δ, blue, n = 15), and R26 N1IC^lox/lox β-catenin^{lox/lox Mx1Cre} (N1IC^Δ/Δ β-catenin^Δ/Δ, light green, n = 9) mice. (F) Kaplan-Meier survival analysis of transplanted mice after last poly(I:C) injection and followed for 115 days. Log-rank (Mantel-Cox) test, ***P value < .001; ****P value < .0001. (G) Phenotypic flow cytometric analysis of transplanted (CD45.2⁺) and induced (eGFP⁺ for N1IC^Δ/Δ, N1IC^Δ/Δ Tcf7^Δ/Δ, and N1IC^Δ/Δ β-catenin^Δ/Δ) BM. Panels on the left depict representative plots of CD4 and CD8 T cells with quantification of absolute numbers on the right. Data are represented as mean ± SEM. Unpaired t-test, ****P value < .0001.