Encodes two divergently transcribed transcription units; a w+mC minigene and a lacZ reporter. en upstream sequence from -2407bp to +188bp (contains no information for patterning) is inserted in between the w+mC minigene and the lacZ coding sequence. Within the upstream en fragment, pairing-sensitive element 1 (PSE1) is flanked by loxP sites, and PSE2 is flanked by FRT sites. The lacZ reporter acts as an enhancer trap, and the en fragment results in 'homing' of the element to the en region.