PU.1 disruption at the CLP stage. (A) B-cell differentiation assays of PU.1^F/F and PU.1^Δ/Δ CLPs in the presence of IL-7. PU.1^F/F and PU.1^Δ/Δ CLPs gave rise to almost equal numbers of CD19⁺IgM⁺ mature B cells after 12 days of culture on OP9 cells in the presence of IL-7. (B) PU.1^Δ/Δ CLP-derived B-cell progeny completely excised floxed alleles. (C) PU.1^F/F and PU.1^Δ/Δ CLPs gave rise to similar sizes of B-cell colonies in response to IL-7 after 12 days in culture. (D) Both PU.1^Δ/Δ and PU.1^F/F CLP-derived B cells rearranged their IgH gene. (E) RT-PCR analyses of B-cell–related genes in PU.1^F/F and PU.1^Δ/Δ B cells (left). IL-7Rα transcripts were quantitated by a real-time PCR analysis (right). Error bars indicate SD. (F) Analysis of spleen B cells developed in PU.1^F/FCD19^Cre/+ mice. PU.1^Δ/Δ CD19⁺ B cells expressed normal levels of IgM and IgD (upper panels). Purified PU.1^Δ/Δ CD19⁺ B cells from PU.1^F/FCD19^Cre/+ mice displayed normal proliferative response to mitogenic agents including anti-IgM antibodies, LPS, and PMA plus ionomycin (P+I) determined by an MTT assay (bottom). *P < .05.