Transgenic construct used to study a Polycomb Group response element (PRE) from the en locus. Generated using the pP{CaSpeR} vector; a mutated 'PRE1' en fragment (extending from -1944 to -1503bp relative to the en transcription start site), which has been altered to contain mutations in pho protein-binding site 'Pho4', has been inserted upstream of the w+mC minigene.