The unstable wDZL mutation of Drosophila is caused by a 13 kilobase insertion that is imprecisely excised in phenotypic revertants.

We have analyzed the lesion in wDZL, a genetically unstable mutant allele of the eye color locus, white, of Drosophila melanogaster. We have cloned the DNA of the white locus region of flies carrying the wDZL allele and find a 13 kilobase insertion not present in the wild-type at the corresponding location. In 12 independent cases examined, reversion to a wild-type eye color phenotype correlates with the excision of a portion of this 13 kilobase insertion, indicating that the insertion is the cause of the mutation. The portion of the insertion that is excised in these eye color revertants is heterogeneous in size but appears to include the central 6 kilobases of the insertion in all cases. Many of these eye color revertants continue to undergo mutation at the white locus, indicating that the residual portion of the insertion in these revertants is sufficient to promote mutations.