Transgenic construct used in a systematic quantitative comparison of Polycomb/Trithorax response elements (PREs) and sequence variants at identical genomic locations. Generated in vivo by integration of donor plasmid sequence into the attP site present in the P{w3'.y+.attP} element. As a result of the integration event, a 1.6kb wild-type vg PRE fragment is inserted directly upstream of a reconstituted w[+] reporter.