Deletion of Gfi1b leads to Rag deregulation. (A) Immunofluorescence of Wt and Gfi1b^−/− AMuLV-transformed cells stained with anti–phospho-H2AX antibodies and counterstained with DAPI. Arrows indicate foci of anti–phospho-H2AX staining. Bars, 10 µM. Data are representative of three independent experiments. (B) Quantitation of data from the experiment in A. The total number of H2AX foci/cell was counted (left) for 400 cells. Data were analyzed using Student’s t test. Right, number of cells with two or more foci/cell, n = 400. Data are representative of 3 biological replicates. (C, left) Cell cycle analysis using PI staining in Wt and Gfi1b^−/− AMuLV-transformed cell lines treated with STI-571 (STI) for the indicated times. (C, right) Percentage of cells with sub-G1 amounts of DNA indicating apoptosis. Error bar represents standard deviation of flow cytometry cell cycle analysis performed in triplicate. Data are representative of three independent experiments. (D) Cell cycle analysis of PI stained AMuLV cells treated with STI-571 and analyzed by flow cytometry. Gfi1bR, Gfi1b^−/− cells reconstituted with a Gfi1b expression vector. Data are representative of two independent experiments.