B-cell development is arrested at the pre–pro-B to pro–B-cell transition in the absence of NF-κB2/p100. (A) Schematic illustration of B-cell development in bone marrow. The expression of characteristic cell surface proteins for successive progenitor stages in mouse B lymphopoiesis are indicated. Lin⁻, negative for the expression of lineage-specific markers (CD3ε⁻CD8α⁻CD19⁻DX5⁻Gr-1⁻Ly6C⁻Mac-1⁻Ter119⁻). (B) Flow cytometric analysis of c-Kit and B220 expression on Lin⁻CD19⁻ wild-type and p100^−/− bone marrow to define pre–pro-B cells (Fr. A). (C) Analysis of CD19 and B220 expression on c-Kit⁺CD43⁺ wild-type and p100^−/− bone marrow to define pro-B cells (Fr. B/C). (D) Analysis of CD19 and CD25 expression on IgM⁻ wild-type and p100^−/− bone marrow to define pre-B cells (Fr. C-prime/D). (E) Analysis of CD19⁺IgM^high immature B cells (Fr. E) in wild-type and p100^−/− bone marrow. Percentages of relevant populations are indicated. Numbers represent mean values (± SD) from 5 mice.