FIG.1.

(A) Generation of the Pax3-FKHR knock-in allele by homologous recombination in ES cells. The top bar represents a physical map of the mouse Pax3 gene encompassing the area around exons 6 and 7 (gray boxes). The middle bar shows the targeting construct in which exon 7 of Pax3 is fused to exon 2 of FKHR (coarsely hatched box) and is followed by a Neo^r gene (finely hatched box) in the opposite transcriptional orientation. A TK selectable marker (black box) is present at the 5′ end. The bottom line represents the targeted Pax3-FKHR allele. Underneath the targeted allele, the positions of the external 5′ and 3′ probes for Southern blot hybridization are indicated. Horizontal arrowheads indicate the positions of primers, used for genotyping and RT-PCR (Table 1). P, PstI; S, SstI; A, ApaI; B, BamHI; X, XhoI. (B) Southern blot with the 5′ probe, showing wild-type (wt) and targeted Pax3-FKHR (+/−) alleles in a wild-type embryo and a Pax3-FKHR heterozygous littermate, respectively. (C) Southern blot with the 3′ probe showing wild-type (wt) and targeted Pax3-FKHR (+/−) alleles. (D) Middle (upper left) and highly chimeric (lower left and right) Pax3-FKHR chimeric mice, showing white spots on the head, belly, and back.