Fig. 1. Generation of a conditional allele of mouse Ptch1. Schematic diagram showing the mouse patched 1 (Ptch1) genomic locus, the targeting vector and the mutant alleles. The top line shows a partial restriction map of the mouse Ptch1 genomic locus on chromosome 13. The mouse Ptch1 genomic locus consists of 23 exons (E1-E23) and only the first two exons are shown for simplicity. The translation start ATG is predicted to reside in the first exon (E1). The regions between the dotted lines represent the 5' and 3' regions of homology used in gene targeting, respectively, and `X' indicates events of homologous recombination. Germline-transmitting chimeric males carrying the targeted Ptch1 locus were mated with FLPe (Rodriguez et al., 2000) mice to remove the PGKneo selection cassette, which is flanked by two FRT sites. The resulting allele is a conditional allele of Ptch1, denoted as Ptch1^c. Ptch1^c/+ animals were mated with a Cre deleter strain [e.g. ß-actin::Cre (Meyers et al., 1998)] to remove sequences between the two loxP sites and to generate a null allele, designated as Ptch1^- or Ptch1^loxP in this study. Matings were set up between Ptch1^loxP/+ animals, and homozygous Ptch1^loxP/loxP embryos display characteristic phenotypes that are indistinguishable from those of the published null alleles of Ptch1 (Goodrich et al., 1997). When the Ptch1 conditional allele is converted into the null allele, ß-galactosidase is brought under the control of the Ptch1 regulatory elements. It should be noted that a small percentage of homozygous Ptch1^c/c animals display embryonic/neonatal lethality, suggesting that the inclusion of lacZ may have affected the expression from the Ptch1 locus.