Loss of both CBP and p300 is highly detrimental for peripheral B cells. (A) Quantitative Southern blot of DNA of FACS-purified splenic B cells from control CBP^flox/flox;p300^flox/flox (Quad) and 3 different quadruple mutant CBP^flox/flox;p300^flox/flox;CD19^+/Cre (Quad mutant) mice. Position of each allele and percent deletion are indicated. (B) Semiquantitative PCR of DNA isolated from quad mutant bone marrow pre-B cells (B220⁺ CD43^–IgM^– purified by FACS) and B cells (B220⁺ purified by FACS) from the spleen and lymph node (LN). Positions of PCR products representing each allele are indicated. PCR tends to overestimate the abundance of the p300^Δ/flox allele. Semiquantitative PCR of DNA isolated from CBP^flox/flox;CD19^+/Cre (C) and p300^flox/flox;CD19^+/Cre (D) B cells as in panel B. (E) Quantification of B-cell progenitors as determined by B220 and IgM flow cytometry (lymphocyte gate) of bone marrow derived from mice of the indicated genotypes. Number of mice is indicated (mean ± SEM). (F) The number of B and T cells per mL of peripheral blood from mice of the indicated genotypes. Number of mice is indicated (mean ± SEM).