Fig. 1. Three Pitx2 alleles. (A) Genomic organization of the Pitx2 locus. Among three expressed Pitx2 cDNA isoforms, Pitx2a and Pitx2b arise by alternative splicing and Pitx2c utilizes an alternative promoter. Arrows above exons indicate transcriptional initiation sites. (B) Pitx2^neo is a hypomorphic allele containing a neo resistance gene within the intronic site of Pitx2. (C) Pitx2 null allele results from the excision of exon 4, which encodes most homeodomain sequences. Black boxes, coding sequences; gray boxes, non-coding sequences; a, Pitx2a-specific exon; b, Pitx2b-specific exon; c, transcriptional start site of Pitx2c isoform; HD, homeodomain.

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