eLife assessment
This valuable paper examines the Bithorax complex in several butterfly species, in which the complex is contiguous and not split, as it is in the well-studied fruit fly Drosophila. Based on genetic screens and genetic manipulations of a boundary element involved in segment-specific regulation of Ubx, the authors provide convincing evidence for their conclusions, which could be strengthened by additional data and analyses in the future. The data presented are relevant for those interested in the evolution and function of Hox genes and of gene regulation in general.