Enhanced expression and activation of STAT3 in the pro-B lineage of BM cells. Freshly prepared BM cells of control mice were first stained with antibodies to CD43, B220, and IgM, followed by permeabilization and staining with primary antibodies either to STAT3 or to phospho-STAT3 and FITC-conjugated secondary antibody to rabbit immunoglobulin. Stained cells were then analyzed by FACS according to surface markers. Pre-B, IMB, and MB were analyzed by gating on CD43^–B220⁺ cells (A). The intensity (MFI) of STAT3 (B, top panels) or pSTAT3 (bottom panels) gated on pro-B (CD43⁺B220⁺), pre-B (CD43^–IgM^–B220⁺), immature B (IMB, CD43^–IgM⁺B220⁺), and mature B (MB, CD43^–IgM⁺B220^high) cells is shown.