Combined deficiencies in Btk and PLCγ2 arrest B lymphopoiesis at a pre-BCR⁺ stage. Bone marrow cells from mice of various genotypes were characterized for their expression of B220 and Igκ/λ (IgL) (A). B220⁺IgL⁻ cells were further analyzed for the expression of CD43, CD25, and MHC class II antigens (B), as well as the cell-surface expression of pre-BCR (C). Numbers indicate percentage of cells in the lymphocyte gate (A), and percentage of B220⁺IgL⁻ cells (B-C). Figures shown are representative of at least 4 independent analyses.