Targeting of the Notch1 gene. (A) Schematic representation of the floxed region of the mouse Notch1 gene. Exons 6 – 8 (* designates a T466A point mutation termed Notch112f described by ) and the HSVTk/Neo cassette were removed by Cre recombinase to generate the Notch1lbdallele. The diagram of Notch1 ECD shows EGF repeats as rectangles and LIN repeats as ovals. The ligand binding domain in EGF repeats 11 and 12 is striped. Amino acids 290–481 were removed by the Notch1lbdmutation. PCR primers 5F, 6R and 9R and the P1415 probe are indicated. B1: BamHI; E1: EcoRI; H3: HindIII. (B) Southern blot analysis of two targeted ES clones (C32 and 132D) by hybridization with probe P1415 after digestion with HindIII or HindIII and EcoRI. (C) Southern blot analysis and PCR genotyping of yolk sac genomic DNA from E9.5 embryos from a Notch1+/lbdheterozygous cross. Genomic DNA was digested with BamHI and probed with P1415. (D) RT-PCR analysis of total RNA from ES cells of the genotypes shown. A hybrid band was obtained from Notch1+/lbdcDNA. (E) E9.5 embryos exhibited defective vascularization of yolk sac and retarded development of Notch1lbd/lbdembryos, but no apparent differences between wild type and heterozygous progeny.