Figure 1A Gene targeting strategy. Left: max genomic locus, targeting construct and max targeted locus. In vitro Flipase (Flip) deletion of the selectable markers lacZ and neo results in a max targeted locus with exons 4 and 5 (coding for bHLHLZip domain) flanked by two loxP sites (fl, flox allele). Arrows represent PCR primers used for genotyping MaxF9 and MaxF10. Right: genomic PCR analysis of tail DNA from F1 mice. PCR products of max wt (430 bp) and max flox (526 bp) alleles. M, 1kb ladder. Flow cytometry analyses of B cell populations (GFP+) in Bone Marrow (BM) and spleen. Single-cell suspensions were prepared from BM or spleen of MaxKO-mb1, MycKO-mb1, DKO-mb1 or heterozygous HET-mb1 control mice, stained with antibodies against the indicated markers, and analyzed by flow cytometry. For BM analysis, cells were isolated from femora and tibiae. Pro-B cells (Ly-6c-CD49b-B220+IgM-CD43+), pre-B cells (Ly-6c-CD49b-B220+IgM-CD43-), immature B cells (Ly-6c-CD49b-B220loIgM+CD43-), and mature B cells (Ly-6c-CD49b-B220hiIgM+CD43-) To identify KO cells (GFP+) all the plots show gated GFP+ B lymphocytes. Flow cytometry analyses of B cell populations (GFP+) in Bone Marrow (BM) Single-cell suspensions were prepared from BM of MaxKO-mb1, MycKO-mb1, DKO-mb1 or heterozygous HET-mb1 control mice, stained with antibodies against the indicated markers, and analyzed by flow cytometry. For BM analysis, cells were isolated from femora and tibiae. Pro-B cells (Ly-6c-CD49b-B220+IgM-CD43+), pre-B cells (Ly-6c-CD49b-B220+IgM-CD43-) Flow cytometry analyses of B cell populations (GFP+) in spleen. Single-cell suspensions were prepared from spleen of MaxKO-mb1, MycKO-mb1, DKO-mb1 or heterozygous HET-mb1 control mice, stained with antibodies against the indicated markers, and analyzed by flow cytometry. spleen follicular B cells (B220+CD23+CD21int), marginal zone B cells (B220+CD23-CD21hi) and immature (newly formed/transitional stage 1) B cells (B220+CD23-CD21lo). To identify KO cells (GFP+) all the plots show gated GFP+ B lymphocytes. E Schematic representation of the gated populations in B and D flow cytometry plots. F, G, H Absolute numbers of B lymphocytes in Bone Marrow (F, G) and spleen (F, H) from MaxKO-mb1 (n=6), MycKO-mb-1 (n=3), DKO-mb1 (n=5) and control HET-mb1 (n=4) mice. Columns represent mean values and error bars standard deviations. *p<0.05, **p<0.01, ***p<0.001. The different B cell populations were identified by flow cytometry Experiment representative of at least three independent experiments. I Cell cycle analysis by flow cytometry of sorted GFP+ pre- and pro-B (B220+IgM-), immature (B220loIgM+) and mature (B220hiIgM+) bone marrow B cells from MaxKO-mb1, MycKO-mb1, DKO-mb1 and HET-mb1 control mice. Mice (n=5) were injected (i.p.) with 200 µg of EdU and analyzed 4 h later. Single-cell suspensions were prepared, stained and sorted. Sorted cells were subsequently stained with propidium iodide for DNA content. Experiment representative of at least three independent experiments. Dotted line is shown for EdU MFI comparison. J Percentage of annexin-V+GFP+ cells in the BM of MaxKO-mb1(n=5), MycKO-mb1(n=3), DKO-mb1 (n=3) and control (n=4) mice. Pro-B, pre-B, immature and mature B lymphocyte subpopulations were identified by flow cytometry as indicated in E. *p<0.05, ***p<0.001. Experiment representative of at least three independent experiments.