Transgenic construct used to study the recruitment of Polycomb group proteins to chromatin. A synthetic Fab element, comprising of of binding sites for Trl, pho and z spaced as in the 219bp Fab-7 minimal PRE segment, but separated by bacterial sequences, plus a 14vp G(A) motif replacing sequence just after the first pho binding site, has been inserted into the pUZ vector, to study the effect on expression of a w[+] minigene in a pairing-sensitive silencing assay.