Cell-intrinsic actions of Ikaros drive basophil differentiation. Bone marrow chimeras were generated by transferring either wild-type or Ik^−/− bone marrow into lethally irradiated Kit^W/Wv recipients. (A) Flow cytometry of c-kit^– FcεRI⁺ basophils in indicated tissues at 4 weeks. (B) Percentage c-kit^– FcεRI⁺ basophils in the indicated tissues of individual chimeric mice. (C) Basophil lineage marker expression on Ik^−/− c-kit^– FcεRI⁺ bone marrow cells. Representative of 4 animals. (D) Fluorescence-activated cell sorted c-kit^– FcεRI⁺ bone marrow cells from Ik^−/− - Kit^W/Wv chimeras express basophil lineage genes, as determined by quantitative RT-PCR. Representative of 4 mice. n.d., not detected. (E) Genotyping of bone marrow basophils sorted from Ik^−/− chimeras. Results represent analyses of 2 recipients of Ik^−/− bone marrow. (F) Bone marrow cells from Ik^+/+ mice were infected with a dominant-negative Ikaros retrovirus (Ik7; MSCV-Ik7-IRES-H-2K^k) or control retrovirus (MSCV-IRES-H-2K^k) prior to culture with IL-3 (5 ng/mL) for 2 weeks. Infected (H-2K^k+) c-kit⁺ FcεRIα⁺ and c-kit⁻ FcεRIα⁺ cells were identified by flow cytometry and (G) basophil:mast cell ratios calculated. (H) H-2K^k+ cells from control and dominant-negative infections were isolated using anti−H-2K^k MACS beads and the expression of lineage-associated genes assessed by quantitative RT-PCR. Data are expressed as the mean of 3 separate analyses ± standard deviation using RNA isolated from same bone marrow culture. Each complementary DNA primer set was run in triplicate. *P < .05; **P < .01; ***P < .001.