Lymphoid development is inhibited in Runx1-excised mice. (A) Representative B220/CD19 and B220/CD43 staining of BM from Runx1^F/F and Runx1^F/F—Tg(Mx1-Cre) mice. CD43 staining is shown for cells gated as IgM^–NKK1.1^–Lin^–. Numbers indicate percentage of cells gated in each respective quadrant. (B) EtBr-stained 3% agarose gels with 3-primer PCR products of Runx1 loci in PB, spleen (Sp), BM, and thymocytes (Th) of mice 154 days after pIpC. Genotypes are as indicated. Control samples are as in Figure 2. The primers do not amplify a product from the Runx1^rd allele. (C) Shown are CD4 and CD8 staining from representative animals of genotypes, Runx1^F/+, Runx1^F/rd, and Runx1^F/rd—Tg(Mx1-Cre). Numbers indicate percentage of cells gated in each respective quadrant. (D) Shown are CD25 and CD44 staining of CD45.2⁺CD4^–CD8^– gated thymocytes from representative animals, as in panel C. (E) Composite of high-speed flow cytometric analysis of BM from representative Runx1^F/F and Runx1^F/F—Tg(Mx1-Cre) mice. Numbers indicate percentages of total BM for indicated populations. Gated populations indicated include IL-7Rα⁺Lin^– (R1), IL-7Rα⁺Lin⁺ (R2) (which includes immature B cells), and CLP (R3), as indicated in Table 1. PI indicates propidium iodide.