FIGURE 1.

Southern analysis of ES cells indicating homologous recombination. A, schematic representation of the wild-type PP5 mouse allele, targeting vector, recombined allele, conditional allele (PP5^flox), and the final PP5 knock-out (PP5^−/−) allele generated by Cre recombinase. loxP sites are indicated by triangles (lox). The relative positions of sequences recognized by restriction endonucleases used in analysis are indicated above the lines. B, diagram showing the position of sites recognized by SpeI (S) and the expected size of DNA produced by SpeI digestion of genomic DNA from ES cells containing wild-type or the targeted PP5 alleles. C, autoradiogram showing the hybridization of a ³²P-labeled 3′ external probe to genomic DNA from ES cells following digestion with SpeI. For Southern analysis, probes that target a region of the PP5 gene outside the targeting vector (indicted by dashed line) were used to ensure the identification of clones in which homologous recombination had occurred. An asterisk designates an example of a positive clone (i.e. clone 256), and an arrow indicates the 6.2-kb band produced by SpeI digestion of genomic DNA in which homologous recombination has occurred.