Inducible Cre/loxP disruption of TβRI in mice. (A) Schematic map of part of the TβRI genomic locus following gene targeting in ES cells and Cre-mediated excision of the selection marker (in ES cells) and exon 3 (in mice), respectively. Arrowheads denote loxP sites, and small arrows indicate the PCR primers used to detect the different recombination events. (B) PCR screening of hematopoietic colonies grown from BM progenitors of induced mice. The PCR primers P1 and P2 and P1 and P3 were used to detect floxed and null alleles, respectively. In this example, all clonogenic progenitors had deleted exon 3 except one, in which one of the floxed alleles was left. (C) TGF-β–induced Smad2 phosphorylation is absent in lineage-depleted (lin^–) BM cells from induced fl/fl Cre mice. Cells were incubated in the presence or absence of TGF-β1 (10 ng/mL), and phosphorylated Smad2 (pS2) was detected by Western blot.