P elements containing a 7 kb DNA fragment from the middle of the Drosophila bithorax complex insert preferentially into the bithorax complex or into the adjacent chromosome regions. This 'homing' property is similar to that reported for the engrailed promoter (Hama, C., Ali, Z. and Kornberg, T. B. (1990) Genes Dev. 4, 1079-1093). The 7 kb fragment does not contain any known promoter, but it acts as a boundary element separating adjacent segmental domains. An enhancer-trap P element was constructed with the homing fragment and the selectable marker flanked by FRT sites. P insertions can be trimmed down by Flp-mediated recombination to just the lacZ reporter, so that the (beta)-galactosidase pattern is not influenced by sequences inside the P element. Twenty insertions into the bithorax complex express (beta)-galactosidase in segmentally limited patterns, reflecting the segmental domains of the bithorax complex where the elements reside. The mapping of segmental domains has now been revised, with enlargement of the abx/bx, bxd/pbx, and the iab-3 domains. The FRT sites in the P elements permit recombination between pairs of elements on opposite chromosomes, to generate duplications or deletions of the DNA between the two insertion sites. Using this technique, the length of the Ultrabithorax transcription unit was varied from 37 to 138 kb, but there was surprisingly little effect on Ultrabithorax function.