Transgene that contains 1. a loxP cassette containing a 210bp fragment containing the minimal Mcp insulator, 2. an FRT cassette containing a 661bp bxd PRE fragment, 3. a w+* marker and 4. the intronless y+mDint reporter. The y reporter is in opposite orientation to the w+* marker (tail-to-tail orientation) and the Mcp fragment is oriented such that the end normally adjacent to iab-4 is closer to the FRT cassette.