Targeted disruption of Mll. (A) Restriction map of wild-type allele (a), targeting vector (b), and mutant allele (c). Exons, PGK-neo, and PGK-tk are depicted as closed boxes. Exons are numbered according to human MLL exons previously reported.40 Arrows indicate the transcriptional direction of PGK-neo. H, HindIII; Hc, HincII; K, Kpn I; RV, EcoRV; X, Xho I. (B) Southern blot analysis of fetal DNA. Genomic DNA isolated from embryos was digested withEcoRV and hybridized with the probe shown in (A). Bands are indicated corresponding to wild-type (12.5 kb) and mutant (3.5 kb) genes. Wild-type (+/+), heterozygous (+/mu), and homozygous (mu/mu) genotypes are shown. (C) RT-PCR analysis of Mll transcripts from F2 fetus. The primers of Mll-5′ and Mll-3′ are indicated by arrows in (D). The numbers of exons are shown in boxes. Deleted exons are depicted as shaded boxes.