Transgenic construct used in an enhancer blocking assay. Contains divergently transcribed w and lacZ reporters. Sequence encoding two tandem copies of the PE enhancer from the twi promoter, a Fab-8 fragment in which both of the two CTCF binding sites have been mutated, and the iab-5 enhancer has been inserted between the two reporters, such that the iab-5 enhancer is proximal to lacZ.