We postulate the presence of a transposable element, designated the L factor, to explain the properties of an unstable X chromosome and its derivatives. These chromosomes generate recessive lethal mutations at high rates, as does a stable X chromosome that has been associated with them for only one generation. The stable X chromosome does not become highly mutable in the absence of the unstable X chromosome, even when autosomes from the unstable stock are present. These facts suggest that the L factor is confined to the X chromosome and that it transposes to other X chromosomes paired with it. We propose the term "homologue destabilization" to denote the change in the stable chromosome brought about by this transposition. The lethal mutations caused by the L factor occur preferentially in the region around the cut wing locus (ct) and are sometimes associated with recognizable chromosome aberrations. The breakpoints of these aberrations are most often in the vicinity of ct, implying that the L factor is located near ct on the unstable chromosome, but it may reside at other sites as well. Alternately, the ct region may simply be a preferred target for the insertion of this transposable element.