Induction of B-cell development in B-Indu-Rag1^fl/fl mice. (A) Strategy of inducible Rag1 expression. Shown is the Rag1 locus of wild-type mice and of Indu-Rag1^fl mice before and after restoration of the initially nonfunctional Rag1 transcription unit by B cell–specific Cre expression and activation. Filled arrowheads, loxP sites; grey boxes, Rag1 exons; CDS, coding sequence. (B) Block of B-cell development in the BM of Indu-Rag1^fl/fl mice. Flow cytometric analysis of c-kit and IgM expression on B220⁺ BM cells from wild-type, Rag1^−/−, and Indu-Rag1 mice heterozygous (fl/wt) or homozygous (fl/fl) for the inactivated allele. (C) Induction of B-cell generation in B-Indu-Rag1^fl/fl mice treated with a single dose of tamoxifen at an age of 8 weeks. Flow cytometric analysis of c-kit and IgM expression on B220⁺ BM cells 10 days after tamoxifen administration. Untreated B-Indu-Rag1^fl/fl mice were included as controls. Numbers in quadrants indicate percentages of cells.