Generation of B-1a cells from the adult BM of B-Indu-Rag1^fl/fl mice. (A) Two groups of wild-type mice and B-Indu-Rag1^fl/fl mice at the age of 8 and 21 weeks were either left untreated or treated twice with tamoxifen. Flow cytometric analysis of peritoneal exudates for IgM and CD5 expression was performed 35 days after initiation of the experiment. Numbers in quadrants indicate percentages of cells. (B) Peritoneal cells from wild-type and tamoxifen-treated B-Indu-Rag1^fl/fl mice were stained for IgM, CD5, Mac-1, and CD125 and analyzed by flow cytometry. Numbers in quadrants indicate percentages of cells. (C) B-1a cells of B-Indu-Rag1^fl/fl mice show N nucleotide additions in all of their IgH chain CDR3. IgM⁺CD5⁺ peritoneal cells from individual induced B-Indu-Rag1^fl/fl mice were sorted, and the IgH chain repertoire was analyzed by single-cell RT-PCR. Nucleotide sequences of representative CDR3 beginning with the conserved cysteine at amino acid position 92 are shown.