Transgenic construct used in an enhancer-blocking assay to identify and characterize an endogenous su(Hw) insulator that is located between the y gene and the achaete-scute complex. Contains the entire y gene, including the wing enhancer (located between -1863 to -2873bp relative to the transcription start site) and the body enhancer (located between -1266 and -1963bp relative to the transcription start site). An FRT cassette containing w regulatory sequences, which include testis and eye enhancers (fragment extends from -1084 to -1465bp relative to the w transcription start site), has been inserted between the y wing and body enhancers. A loxP cassette containing a 2kb insulator fragment from the y-ac intergenic region has been inserted between the y body enhancer and y transcription start site (at position -893 relative to the transcription start site). A gypsy insulator has been inserted downstream of the y coding region, followed by the w^+mC minigene.