Transgenic construct used in a pairing-sensitive repression (PSR) assay. Generated by inserting a 4 kb PstI fragment including upstream sequence and 5' transcription unit of ph-p (nucleotides 70,027 to 66,143 of AE003423) into pCaSpeR4 (which contains the w+mC minigene). The orientation of this fragment relative to the w ORF is the reverse of its normal orientation relative to the ph-p transcription unit.