Efficient inactivation of CBP^flox or p300^flox in mouse B cells does not result in lymphoma. Quantitative Southern blot of genomic DNA isolated from purified splenic B cells for CBP^flox/flox;CD19^+/Cre (A) and p300^flox/flox;CD19^+/Cre (B) mice. Nonrecombined (flox) alleles, recombined (Δflox) alleles, and percent deletion are indicated. (C) Western blot of cell lysates from purified splenic B cells using CBP-specific (top panel) and p300-specific (bottom panel) antisera. Genotypes, 200-kDa markers, and position of CBP and p300 are indicated. (D) Indirect immunofluorescence of control and CBP and p300 null purified splenic B cells using CBP- and p300-specific antisera. Genotypes are indicated. Nuclear DNA stained with DAPI. Survival curves for CBP^flox/flox;CD19^+/Cre (E) and p300^flox/flox;CD19^+/Cre (F) and control mice. Age of mice (days) and percent surviving are indicated. Tick marks indicate mice that were censured (killed for experimentation) or still alive at the end of the study. Curve comparisons by 2-tailed log-rank test, number of mice of each genotype, and P values are indicated.