Specification defect of the lymphoid lineage in tPtch^−/− mice is dependent on stromal BM cells. Lin⁻ BM cells (400 000) from vehicle- and tamoxifen-treated control animals or day-19 tPtch^−/− mice were injected intravenously into lethally irradiated Rag-2^−/−γc^−/− mice. After 7 weeks, peripheral blood cells were analyzed by flow cytometry (A). Cells within the lymphocyte gate (left panels) were analyzed by staining with antibodies against either CD4 and CD8 (middle panel) or B220 to identify T and B cells, respectively. Nonreconstituted Rag-2^−/−γc^−/− mice served as negative control (top row). The presence of inactivated Ptch alleles in T and B cells from reconstituted Rag-2^−/−γc^−/− mice was detected by quantitative PCR (B). Data are representative of 3 independent experiments (each experiment consisted of the transfer of Lin⁻ BM cells derived from 1 vehicle-treated Ptch^flox/floxERT2^+/−, 1 tamoxifen-treated Ptch^flox/floxERT2^−/−, and 1 tPtch^−/− mouse).