Homozygous In(lO)lTRk mice contain a paracentric inversion within Chromosome (Chr) 10 and exhibit a pygmy phenotype, suggesting that the distal inversion breakpoint is within the pygmy (pg) locus. In order to obtain the pygmy gene by positional cloning procedures, In(10)17Rk DNA was subjected to RFLP analysis with single-copy probes derived from the wild-type pygmy locus. This analysis identified a DNA polymorphism in the DBA/ 2J mouse strain on which the In(lO)17Rk mutation was originally induced. A detailed characterization of this polymorphism revealed the presence of a novel, tandemly repeated DNA element.
Copy number estimation experiments indicate that there are approximately 100,000 copies of this element in the haploid DBA/2J genome. PCR typing studies revealed the presence of the repeat at the pygmy locus of 6 of the 18 Mus domesticus strains analyzed. The absence of the repeat from the pygmy locus of 12 strains of the M. domesticus species and from the M. caroli, M. spretus, M. castaneus, and M. molossinus species suggests that the repeat could serve as a strain-specific hybridization probe in genetic mapping studies. Finally, the novel tandem DNA repeat is conserved in both rat and human genomes as indicated by Southern hybridization experiments.