Transgenic construct used in an enhancer-blocking assay to study the mechanism of chromosomal boundary action. A 370bp fragment of the gypsy transposon, containing 12 degenerate binding sites for su(Hw), has been inserted into P{wRW+we.Fab-8} upstream of the w enhancer. A 1.7kb Abd-B-Fab-7 region has been inserted in between the w enhancer and w coding sequences.