Fig. 1 Targeting of the VEGF gene. a, Modification of the VEGF gene. Top box, Targeting vector pPNT. VEGF120 and wildtype VEGF allele (WT VEGF). Middle box, Homologously recombined VEGF allele (VEGFneo/tk). Bottom box, Modified VEGF120 allele after Cre-excision of the floxed neo/tk cassette. Filled boxes, numbered exons; grey shaded bars, intron sequences; lettered bars external hybridization probe A (0.65-kb Sma1–BglI fragment) and internal hybridization probe B (1.7-kb NcoI–EcoRI fragment). The observed sizes of the diagnostic restriction fragments, used to distinguish the wild-type and mutant alleles, correspond to their expected sizes. b, Southern blot analysis of EcoRI-digested genomic DNA from VEGF+/+ and VEGFneotk/+ ES cell clones using the external 5’probe A, which generates a 7.5-kb wild-type fragment and a 2.5-kb VEGFneotk fragment. c, Southern blot analysis of BamHI-digested genomic DNA from VEGF+/+, VEGF+/120 and VEGF120/120 mice using the external 5’probe A, which generates a 9.5-kb wild-type fragment and a 7.8-kb VEGF120 fragment. d, RNAse