Transgenic construct used to study insulator function in an enhancer blocking assay. Contains a 3kb SalI-BamHI genomic fragment containing the y regulatory region fused to a 5kb BamHI-BglII genomic fragment containing the y coding region. An FRT cassette containing w regulatory sequences from -1084 to -1465bp (contains the testis and eye enhancers) has been inserted between the y wing and body enhancers at position -1868 relative to the y transcription start site. A loxP cassette containing a 325bp fragment of 1A-RE insulator sequences has been inserted at position -893 relative to the y transcription start site. A w^+mC minigene is present downstream of the y coding sequence.