THE interactions between enhancers and promoter elements that control gene expression are generally considered to act in cis only, but genetic studies suggest that they can also function in trans between non-contiguous DNA molecules. Termed transvection(1), such trans interactions have been proposed to be responsible for several examples of intragenic complementation in Drosophila(1-9). Transvection is thought to depend on the physical proximity of sister chromosomes(10,11), because it is inhibited when chromosome rearrangements reduce the pairing of homologues(1,3,5). This led to the suggestion that transvection occurs enhancer elements on one chromosome regulate expression on the other(7,12), with the pairing dependence resulting from a need for proximity between the two copies of the gene. Here we have analysed the levels of transcription from both alleles of the Drosophila Ultra-bithorax (Ubx) gene, and report that the predictions of this simple model are not supported. Our findings indicate a more complex level of trans regulation that may have implications for the aetiology of genetic disorders that are influenced by chromosome rearrangements.