Fig. 1. Fgfr1 targeting. (A) Alleles introduced by gene targeting. (B) Partial cDNA knock-in approach: Fgfr1 exons 8-17, encoding the transmembrane and cytoplasmic domains, were replaced with a pseudoexon encoding the corresponding region of Fgfr1^wtKI or Fgfr1^Frs. Partial cDNAs were spliced at the 5' end into exon 8 and at the 3' end into exon 17, upstream of the endogenous polyadenylation sequence. (C) To generate the null allele, the first exon common to all reported Fgfr1 splice variants (exon 4) was flanked with loxP sites and excised in vivo by Meox2-Cre (Tallquist and Soriano, 2000). RT-PCR from embryonic RNA confirmed the generation of a stop codon soon after the exon3-exon5 splice junction (data not shown). (D) Southern blots verifying correct targeting of all alleles in ES cell clones. Abbreviations: A, ApaI; H, HindIII; KI, knock-in; Nd, NdeI; Nh, NheI; P, PstI; Rv, EcoRV; T, Tth111I; X, XbaI; ext, external; int, internal. (E) Relative Fgfr1 mRNA levels in homozygous embryos, assessed by semi-quantitative RT-PCR. Each point is the mean of triplicate reactions for a single embryo (±s.d.).