Encodes two divergently transcribed transcription units; a w+mC minigene and a lacZ reporter. An en fragment containing 8kb of upstream sequence, with a deletion of 'pairing-sensitive element' (PSE) 2 (-576 to -395bp relative to the en transcription start have been deleted), the en promoter, and 188 bases of the untranslated leader (from -8kb to +188bp) is inserted in between the w+mC minigene and the lacZ coding sequence.