Proliferation of immunoglobulin class–switched B cells. Flow cytometric analysis of BrdU-labeled B220⁺ IgM^- IgD^- spleen cells from mice fed with BrdU in their drinking water continuously for 1, 3, or 5 days. Three mice of each indicated genotype were analyzed at each time point. The left panel shows the proportion of B220⁺ IgM^- IgD^- cells labeled with BrdU after 3 days. The right panel shows the total number of BrdU-labeled B220⁺ IgM^- IgD^- cells per spleen after 1, 3, and 5 days.