Targeted inactivation of the rap1b gene. (A) The murine rap1b gene consists of 6 coding (bands) and 1 untranslated exon (3′ UTR, open box). (B) The targeting vector contains 7.8 kb of genomic DNA flanking the neomycin-resistance cassette (neo). TK, thymidine kinase. (C) After homologous recombination, the neo cassette replaces the complete coding sequence of the rap1b gene. (D) Southern blot analysis of mouse tail DNA from heterozygous intercrosses digested with SspI and KpnI using a probe (P) that detects 5.5-kb and 9-kb fragments in the wild-type and knockout allele, respectively. (E) Western blot analysis of protein expression in platelets of indicated genotype. (F) Morphology of E15.5 wild-type (+/+) and Rap1b-null (–/–) embryos. Scale bar: 1 mm. E, EcoRI; H, HinDIII; K, KpnI; S, SspI.