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 EcoRI-PstI fragment containing 1A-RE insulator sequences is inserted, in reverse orientation, at position -893 in the y regulatory sequences.