Zfx deletion results in a block in early B-cell development. (A) The expression of Zfx in sorted B-cell populations from the BM of wild-type mice. Shown are normalized Zfx expression levels relative to the Hardy fraction B sample, as determined by qPCR (mean ± SD of triplicate reactions). (B) The analysis of Hardy fractions in the BM of control and Tie2-Cre⁺Zfx^flox/y CKO mice at 4 to 5 weeks. The percentages of fractions A-C′ (B220⁺ CD43⁺), D-F (B220^hi CD43⁻), C (late pro-B, HSA^int BP-1⁺), and C′ (large pre-B, HSA^high BP-1⁺) in the total BM are shown for a representative staining. The ratio of Fr. C′/Fr. C populations was 4 in control and 1 in CKO BM (P = .003, average of 4 mice per group). (C) The analysis of Hardy fractions in the BM of control and Mb1-Cre⁺Zfx^flox/y CKO mice at 4 to 6 weeks. The percentages of fractions A-C′, C, and C′ are indicated (mean ± SD of 6-7 mice per group). (D) Absolute numbers of fractions C and C′ in control and Mb1-Cre⁺Zfx^flox/y CKO mice (mean ± SD of 6-7 mice per group). (E) Excision efficiency of Zfx in sorted Hardy fractions of Mb1-Cre⁺Zfx^flox/y CKO mice as determined by genomic PCR. (F) The expression of intracellular μ heavy chain in control and Mb1-Cre⁺Zfx^flox/y CKO mice. Intracellular IgM histogram profiles of B220⁺ CD43⁺ CD25⁻ late pro-B cells and B220⁺ CD43⁺ CD25⁺ large pre-B cells are shown; the IgM⁺ fraction is indicated (mean percentage ± SD of 3 mice per group). Gray histograms represent isotype control. (G) Proliferation of large pre-B cells in control and Mb1-Cre⁺Zfx^flox/y CKO mice. Mice were injected intraperitoneally with BrdU 50 minutes before analysis. The fraction of BrdU⁺ cells in the B220⁺ CD43⁺ CD25⁺ large pre-B cells of individual mice is shown.