The majority of B-1 cells lack Blimp-1 expression. Representative FACS plots showing YFP (Blimp-1) and CD138 staining among CD19^neg IgM⁺CD43⁺ B-1PC and CD19⁺IgM⁺ IgD^lo/neg CD43⁺ B-1 cells in (A) BM and (B) the spleen. Values shown in all FACS plots represent percentage within parent population. (C) Representative FACS gating for peritoneal cavity B-1 cells. (D) Blimp-1 YFP expression among B-1 cells in BM, spleen, and peritoneal cavity was defined as either negative, intermediate, or high. FMO, fluorescence minus one control stains for IgM (C) and CD5 (D), respectively. (E) Frequency ± SD of Blimp-1 YFP high, intermediate, and negative cells among B-1a (left) and B-1b (right) cells in the spleens, BM, and peritoneal cavities of C57BL/6 mice (n = 4–10). Results are pooled from at least three independent experiments.