Proportions of B-cell progenitors in CBP^flox/flox;CD19^+/Cre and p300^flox/flox;CD19^+/Cre mice are normal but p300 is essential before or during the pro-B stage. (A) Representative B220 and IgM flow cytometric analyses of lymphocyte gated bone marrow cells from the indicated mouse genotypes. Percentage of each major subtype is indicated. (B-C) Quantification of bone marrow B-cell progenitors by B220 and IgM flow cytometry as in panel A. Pro-B and pre-B cells (B220^lo IgM^–), immature B cells (B220^int IgM⁺), and mature recirculating B cells (B220^hi IgM⁺). Number of mice of each genotype is indicated. Mean ± SEM. (D) Recombination of the p300^flox allele in FACS-purified pro-B (B220⁺ CD43⁺ IgM^–) and pre-B (B220⁺ CD43^–IgM^–) cells from a representative p300^flox/flox;Mx-Cre mouse with efficient deletion 6 months after inducing Cre expression with dsRNA. (E) Number of pro-B and pre-B cells in dsRNA-treated wild-type (WT; n = 4) and p300^flox/flox;Mx-Cre mice (n = 2) as in panel D (mean ± SEM). (F) Representative histogram showing B220⁺ bone marrow cells as in panels D-E.