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, followed by an FRT cassette containing a 0.86 kb Fab-7 insulator fragment in reverse orientation is fused upstream of the y coding region. The w gene is present downstream of the y coding region. A loxP cassette containing a 0.86 kb Fab-7 insulator fragment in reverse orientation has been inserted between the y and w coding regions.