B-cell development is arrested at the pre–pro-B-cell stage in Stat5a/b^null/null survivor mice. (A) Percentages of pre–pro-B, early pro-B, and late pro-B cells in bone marrow and (B) mature B cells in spleen and lymph node of 5 Stat5a/b^null/null survivors compared with 2 Stat5a/b^null/+ and 2 Stat5a/b^+/+ controls. Due to the small body size, bone marrows were pooled and therefore did not allow generation of error bars. (C) Schematic model for maturation of B-cell developmental fractions A-F. As indicated, individual maturational stages were distinguished by differential surface expression of B220, CD43, CD19, BP-1, IgM, and IgD. The different blocks in Stat5a/b^ΔN/ΔN and Stat5a/b^null/null mice are indicated by vertical lines. CLP indicates common lymphoid progenitor.