Failure of in vitro expansion of pro-B cells from Sox4 conditional knockout mice elucidates the essential role of Sox4 in early B-cell development. Cells were cultured in the presence of OP9 stromal cells and IL-7 and analyzed by flow cytometry. (A) Analysis of cultured embryonic fetal liver cells from Sox4^fl/+Vav-Cre or Sox4^fl/flVav-Cre mice for the presence of B220^lowCD24⁺BP1⁻ (fraction B) pro-B cells and B220^lowCD24⁺BP1⁺ (fraction C) pro-B cells. (B) Analysis of cultured total bone marrow cells from Sox4^fl/+Vav-Cre or Sox4^fl/flVav-Cre mice for the presence of B220⁺CD19⁺ B cells. (C) Analysis of cultured pre-pro-B cells sorted from the bone marrow of Sox4^fl/+Vav-Cre or Sox4^fl/flVav-Cre mice for the presence of B220⁺CD19⁺ B cells. Hardy’s nomenclature⁴  is used, for example, pre-pro-B (fraction A), pro-B (fractions B and C), pre-B (fraction D), immature B (fraction E), and mature B (fraction F). Numbers indicate percentages corresponding to the cells outlined. All data are representative of 3 independent experiments.