The X-linked singed locus is one of several in Drosophila that controls the formation of bristles on the adult cuticle. Males that are hemizygous for a mutant singed allele have bent, twisted bristles that are often much reduced in size. Several P element insertion mutations of the singed locus have been characterized, and some have been shown to revert to the wild-type allele by excision of the inserted element. What conditions must be present to allow such reversions to occur?