Fig. 1. Analysis of Cre-mediated inactivation of Fgf8 in MHB KO mutants. (A) Schematic representation of the mutant alleles carried by En1^Cre/+, Fgf8^flox/null embryos (MHB KO mutants) and a reporter gene used to detect Cre activity. Cre-mediated recombination of the R26R reporter gene deletes the neo sequences flanked by loxP sites, and allows expression of lacZ. Cre-mediated recombination of Fgf8^flox deletes crucial exons, creating the Fgf8^null allele. (B,C) Dorsal views of embryos at the somite stages (s) indicated, assayed for En1 expression by RNA whole-mount in situ hybridization (B), or for Cre-mediated recombination by staining for ß-galactosidase (ß-GAL) activity (C). (D) Control and MHB KO mutants hybridized with a probe that detects Fgf8 exon 3. Embryos at the 7 somite stage are shown in lateral view only; those at the 11 somite stage are shown in both lateral (left) and dorsal (right) view. Red asterisks indicate the regions in which Fgf8 expression is detected in control, but not in mutant embryos. frt, frt site present in the Fgf8^flox and Fgf8^null alleles, as described by Meyers et al. (Meyers et al., 1998); pro, promoter; s, somite stage.

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