The Fab-7 boundary functions to ensure the autonomous activity of the iab-6 and iab-7 cis-regulatory domains in the Drosophila Bithorax Complex from early embryogenesis through to the adult stage. Although Fab-7 is required only for the proper development of a single posterior parasegment, it is active in all tissues and stages of development that have been examined. In this respect, Fab-7 resembles conventional constitutive boundaries in flies and other eukaryotes that act through ubiquitous cis-elements and trans-acting factors. Surprisingly, however, we find that the constitutive activity of Fab-7 is generated by combining sub-elements with developmentally restricted boundary function. We provide in vivo evidence that the Fab-7 boundary contains separable regions that function at different stages of development. These findings suggest that the units (domains) of genetic regulation that boundaries delimit can expand or contract by switching insulator function off or on in a temporally regulated fashion.