Transgenic construct used to characterize Polycomb response elements (PREs) at the inv locus. Contains a mini-w marker and an en-lacZ reporter in which en enhancer sequences (8kb fragment from which a 2kb fragment containing en PRE1 and PRE2 has been deleted) and an FRT cassette containing a predicted PRE fragment from the inv locus are fused upstream of the en promoter and lacZ.