Lck-Cre–mediated conditional Cbfb-MYH11 expression in adult thymus. (A) The Cbfb^56M allele contains an insertion in intron 4 that includes exons 5 and 6 of Cbfb flanked by 2 lox sites. It also contains the knocked-in MYH11 cDNA in exon 5. Cre-mediated excision of the exons 5/6 cassette led to the restoration of the knock-in Cbfb-MYH11 fusion (Cbfb^56M excised allele). PCR primers used for quantitative PCR to determine the efficiency of Cre-mediated deletion were: e5 and e6 primers for the Cbfb^56M allele; neo primers for both the Cbfb^56M and the Cbfb^56M excised alleles. (B) Total thymocyte numbers from Tg(Lck-Cre) and Tg(Lck-Cre)Cbfb^+/56M embryos at E17.5 (n = 4 for each genotype) and E18 (n = 3 for Tg(Lck-Cre) and n = 6 for Tg(Lck-Cre)Cbfb^+/56M). (C) Representative contour plots of cell surface marker expression of thymocytes from Tg(Lck-Cre)/Cbfb^+/56M and littermate control Tg(Lck-Cre) embryos at E18.0. The cells were stained with CD4, CD8, CD44, and CD25. The top panel shows CD4 and CD8 distribution, and the bottom panel shows CD44 and CD25 staining of DN cells.