Impaired Ig CSR in Mule-deficient B cells. (A) Purified splenic B cells from mice of the indicated genotypes were stimulated with either LPS (left) or LPS plus IL-4 (right) for 3–5 d and analyzed by flow cytometry to detect IgG3⁺ class-switched B cells (left) and IgG1⁺ class-switched B cells (right). Numbers are the percentage of class-switched B cells among gated B cells. Results are representative of three independent experiments involving two to five mice per genotype. (B) Purified splenic B cells were labeled with CFSE and stimulated for 4 d with LPS plus IL-4. Cell divisions tracked by CFSE were analyzed by flow cytometry, with each peak indicating one cell division. Results depicted are representative cell division profiles of littermate control B cells (left), BMKO B cells (middle), and the overlay of control cells over BMKO cells (right). Results from A and B are representative of two independent experiments involving two to three mice per genotype.