BCR expression in Igh^CGG mice. (a) Left: Flow cytometry of splenic B cells (B220-gated) from Igh^CGG/a mice (where the Igh^CGG allele carries the IgM^b allotype and the WT allele carries the IgM^a allotype) compared with Igh^a/b mice (where both IgM^a and IgM^b alleles are WT). Right: Flow cytometry of splenic B cells from Igh^CGG+ and WT mice showing allelic exclusion of endogenous Cκ and Cλ chains. One plot representative of three mice is shown for each. Numbers within flow plots indicate percentage of cells in the quadrant. (b) SIgM and IgD expression on WT and Igh^CGG/+ follicular B cells. Gated on CD19⁺/human Cκ⁺/CD93⁻/CD23⁺ cells for Igh^CGG/+ mice or on CD19⁺/CD93⁻/CD23⁺ cells for WT. WT T cells are shown as a negative control. Data from independent experiments are shown on the right. Each symbol represents one mouse. **, P ≤ 0.01 (unpaired t test). gMFI, geometric mean fluorescence intensity. (c) Western blot for IgM in naive B cells from Igh^CGG/+ mice. Anti-IgM signal for WT and anti-CGG B cells is shown at two different exposures. The relevant band is indicated at ∼70 kD. The β actin loading control is shown below. Note the absence of an ∼95-kb band, the expected size of the uncleaved Vκ/V_H construct. Data are representative of two experiments.