Comparison of Polycomb response elements (PREs) by ΦC31 site-specific integration. (A) The plasmid pKC27 carries the PRE of interest, the 5' end of the miniwhite (mw) reporter gene including the first exon and 195 bp of the first intron and the phage ФC31 attB recognition site. The genomic landing site carries an attP site followed by a genomic DNA fragment of the white gene containing the last 205 bp of intron 1 and the remaining downstream exons. (B) Correct site-specific integration is detected by the expression of a functional mw reporter gene. (Top right) flies carrying only the mw reporter have red eyes. (Bottom right) The mw reporter is repressed in the presence of a PRE, resulting in a decrease in eye coloration.