Proposed mechanism for exon duplication and long-range PCR strategy. (A) Impaired recombination between repetitive elements in intron 2 (solid line) and intron 3 (dotted line) of sister chromatins would generate one strand with the duplication of exon 3 and the other strand with the deletion of exon 3. (B) Schematic of the long-range PCR strategy. Primer pair in exon 3 (F indicates forward primer at the junction of exon 3 and intron 3; R, reverse primer at 5′ of exon 3) was designed to generate a PCR product only in the presence of duplicated exon 3 from the mutant KitlSl-20J genomic DNA. (C) Long-range PCR analysis of genomic DNA. An approximately 7-kb product (arrow) generated from DNA isolated from heterozygous KitlSl-20J mice (lane 2) not amplified from wild-type DNA (+/+, lane 3) supports impaired recombination as the mechanism of this mutation. M indicates marker lane.