Fig. 1. Pitx2 alleles and Pitx2 isoform expression in developing teeth. (A) Summary of Pitx2 isoforms. Numbered boxes represent exons and black boxes the homeodomain. (B) Pitx2 abc^creneo targeting strategy. At the top is a wild-type allele, the targeting vector is in the middle and at the bottom is the targeted allele. Numbered boxes represent exons and lines the intervening introns. Black shaded areas are homeobox. p1 and p2 are two alternate promoters located upstream of exon 1 and exon 4. On the right are Southern blots with 5' and 3' flanking probes. (C) Five Pitx2 alleles: Pitx2a and Pitx2b isoform-specific deletions, ab^hypoc and ab alleles (Liu et al., 2001). The abc^null allele is a homeobox deletion and a Pitx2-null allele (Lu et al., 1999) and c allele is and isoform-specific deletion of Pitx2c. (D) lacZ staining of ab^+/– embryo showing expression in oral ectoderm (arrow). (E,F) Coronal (E) and parasagittal (F) sections through molar cap stage tooth showing lacZ expression in epithelial components and enamel knot (arrows). (G) Whole-mount in situ with Pitx2c probe showing expression in oral ectoderm (arrow). (H,I) Coronal (H) and parasagittal (I) section in situ with Pitx2c probe showing expression in epithelium of cap stage tooth. (J) RTPCR of Pitx2 isoforms in oral and dental epithelium. Specific amplified bands for each isoform (arrow). + indicates inclusion of reverse transcriptase;–is control without reverse transcriptase. md, mandibular process; mx maxillary process.