Early block of B-lineage commitment in the BM of tPtch^−/− mice. BM cells from vehicle- and tamoxifen-treated control animals and of day-19 tPtch^−/− mice were analyzed by flow cytometry using antibodies against B220, CD43, and IgM (middle and right panels) to distinguish pro-B (fraction I), pre-B (fraction II), and the pool encompassing immature, mature, and recirculating B cells (fraction III) within the lymphocyte gate defined by the SSC/FSC plots (left panels). Data represent 4 independent experiments (each experiment consisted of 1 vehicle-treated Ptch^flox/floxERT2^+/−, 1 tamoxifen-treated Ptch^flox/floxERT2^−/−, and 1 tPtch^−/− mouse).