Fig. 2. PDGFR conditional allele and analysis of Cre efficiency. (A) The wild-type PDGFR locus, the targeted floxed allele and the resulting allele after Cre-mediated inactivation of the PDGFR. The deleted region of genomic DNA in the fl allele is identical to that deleted in the PDGFR null allele R4 (Soriano, 1997). Black boxes represent exons. The floxed allele contains the neo cassette (gray box). Red arrowheads represent loxP sites, and the small black rectangle represents the probe used in Southern blot analysis. (B) Southern analysis of tissues from E13.5 PDGFR^fl/fl; Wnt1Cre⁺ embryos. Genomic DNA was digested with NheI and probed with fragment indicated in A. Lane 1, DNA from yolk sac; lane 2, DNA from cranial tissues. (C,D) Transverse sections through first branchial arch region of E9 embryos stained for ß-galactosidase activity. (C) ROSA26R^+/-; Wnt1Cre⁺; section was counterstained with nuclear Fast Red. (D) PDGFR^fl/fl;Wnt1Cre⁺;ROSA26R^+/-; section was not counterstained. Note that D contains a similar distribution of ß-galactosidase-expressing cells in the branchial arches when compared to the distribution of ß-galactosidase-expressing cells in C. (E) PDGFR cell-surface expression histogram. Cells from E10.5 embryos were isolated from the branchial arches, stained for PDGFR expression, and analyzed by flow cytometry. Red profile, cells stained with only secondary antibody; gray profile, cells from PDGFR^fl/fl embryo; black profile, cells from PDGFR^fl/fl;Wnt1Cre embryo.