Loss of Notch1 expression in TgE mice results in increased IgM⁺ B cells in the spleen. (A) Total cells isolated from the spleen of each indicated genotype were stained with B220 and IgM fluorochrome-conjugated antibodies and analyzed by flow cytometry. Live cell populations, designated by forward and side scatter properties, were plotted with IgM and B220, and a representative plot is shown for each genotype. CD19-Cre–mediated deletion has been demonstrated previously to have an efficiency of approximately 80% in the bone marrow and 90% to 95% in the spleen.¹⁶  (B) The average percent of B cells based on B220 staining for each genotype is shown plus or minus SE. (C) The average percentage of B cells that have IgM on the cell surface (%B220) as well as the percentage of total spleen cells that are IgM⁺ (% total) for each genotype is shown plus or minus SE. The percentage of B220 cells expressing IgM (%B220) is calculated to account for differences in total B cells between the genotypes. Results from 4 independent experiments are shown. *P = .04, **P = .05 by t test. Notch1^lox/lox, n = 5 mice; Notch1^lox/lox CD19^+/Cre, n = 4; TgE Notch1^lox/lox, n = 7; TgE Notch1^lox/lox CD19^+/Cre, n = 8.