P{w^RW+we} is designed for use in an enhancer-blocking assay, which can be used to define and characterize chromatin domain boundary elements. P{w^RW+we} contains the w transcription unit plus a 1107bp minimal BamHI-HindIII w enhancer fragment that drives expression in the eye and testis. Unique cloning sites have been introduced upstream (KpnI) and downstream (NotI and XhoI) of the enhancer. A 540bp EcoRI-BamHI fragment of the scs' element has been inserted downstream of the w coding region to reduce position effects from the 3' side. Sequence to be tested for enhancer-blocking activity can be inserted in between the promoter and upstream enhancers, to analyse the effect on pigment expression in the eye and testis compared to the level seen in flies carrying the original P{w^RW+we} element without the inserted sequence.