Gene targeting strategy to introduce AML1/EVI1 chimeric gene into the murine AML1 locus. (A) Schematic presentation of partial murine Aml1 genomic locus (top line), replacement vector containing a partial human AML1/EVI1 cDNA, a neomycin resistance cassette (Neo) for positive selection and a diphtheria toxin-A cassette (Dta) for negative selection (middle line), and the targeted allele (bottom line). (B) Southern analysis of wild-type (+/+) and AML1/EVI1 knock-in (KI) ES cell clones. For detecting homologous recombination, the murine Aml1-specific probe indicated in the lower line of panel A was used. (C) RT-PCR analysis on E12.5 fetal liver cells from wild-type (+/+) and KI embryos. AML1/EVI1 mRNA was amplified using primers A and B indicated in the lower line of panel A. Amplification was also performed for β-actin mRNA as a control for the presence of amplifiable RNA.