Transgenic construct used to study the properties of the Fab-7 insulator, by analyzing its effects on y and w expression in an enhancer blocking assay. A fragment which is flanked by two RS(I-SceI) sites and which contains the y wing and body enhancers, with the w eye enhancer inserted between them, is fused upstream of the y coding region. The w gene is present downstream of the y coding region. A loxP cassette containing a 2.7kb spacer (from the e(y)3 gene), followed by an FRT cassette (at -893 relative to the y transcription start site) containing a 0.86 kb Fab-7 insulator fragment have been inserted between the fragment containing the y and w enhancers and the y coding region.