Dynamic expression of Gfi1b during B-cell development. (A) Cells from different tissues of the hematopoietic system of adult wt or Gfi1b^+/GFP mice or from fetal livers of wt, wt/KI (Gfi1b^+/GFP or KI/KI (Gfi1b^GFP/GFP) embryos, respectively, were stained with antibodies against the surface markers B220 and Mac-1 and subjected to FACS analysis. Mac-1–negative, B220-positive living cells were electronically gated and analyzed for GFP expression. (B-D) Bone marrow cells from adult wt or wt/KI (Gfi1b^+/GFP) mice were stained with antibodies against B220 and IgM (B), CD43 (C), or CD25 (D) and analyzed by FACS (left panels). Fractions of cells representing the different maturation stages of B cells (A indicates pre-pro-B; B/C, pro-B; C, large pre-B; D, small pre-B; E, immature B; F-F, mature B cell)^17,18  were electronically gated as indicated (left panel) and analyzed for GFP expression (histograms in right panels). (D) B220-positive cells were gated for high and low CD25 expression, and cell size distribution was separated by forward angle light scatter analysis (right panel, left histograms). GFP fluorescence histograms for large and small cells (L indicates large B-cell; S, small B-cell; LC, large cycling B cell; SR, small resting B cell) were generated (right panel, right histograms) as indicated.