Fig. 1. Targeting of the Cdx1 RARE. (A) Schematic representation of the 5' genomic region of the Cdx1 locus, targeted allele (targeting vector sequences shaded) and anticipated Cre-recombination product. The sequences external to the targeting vector used for screening initial recombinants are indicated below the targeted allele, as is the internal probe used to screen for Cre-catalyzed recombination. (B) A multi-enzyme Southern blot demonstration of the predicted targeting event using the internal probe denoted in A with the restriction endonucleases used indicated above each lane. (C) Characterization of Cre-mediated recombination. Primers (arrows in A) were used to amplify over the RARE region by PCR. Products were resolved by agarose gel electrophoresis and characterized by Southern blot analysis using oligonucleotide probes specific for wild-type or recombination products. As anticipated, the wild-type (WT, lower panel) band is present in all samples with the exception of the negative control (lane 7), while the Cre-generated product is observed only in mice from the appropriate mating (lane 2) or from a cell line transfected with a Cre expression vector (lane 6). S, SacI; H, HindIII; R, EcoRI; K, KpnI; X, XhoI.