Comparison of Fab-7 and vg Polycomb response elements (PREs) at four landing sites. (A) Eye phenotype of transgenic fly lines; 5-day-old male flies are shown. In each row, flies (top) heterozygous and (bottom) homozygous for each transgene are shown at all four landing sites. (Top row) No PRE: mw reporter alone; (middle row) Fab-7 PRE: 1.6 kb of the Fab-7 PRE flanking the mw reporter; (bottom row) vg PRE: 1.6 kb of the vg PRE flanking the mw reporter. (B) Quantification of eye pigment in heads of flies shown in (A). Pigment levels are expressed as percentage of wild-type pigment. 50 heads per assay were used, of an equal mix of male and female flies, aged 5 days. Mean and standard deviation of two biological replicates are shown. (C) Repression index w calculated from mean pigment levels in (B) as no PRE/PRE for each transgenic line at each landing site for heterozygotes and homozygotes. The graph shows the maximum, minimum and mean (horizontal line) repression index at four landing sites for each PRE, in (left) heterozygotes and (right) homozygotes. Values <1 indicate activation. (D) Pairing-sensitive silencing (PSS) was calculated from mean pigment levels in (B) as heterozygote/homozygote for each transgenic line, normalized to the same ratio calculated for the 'no PRE' control line at the same landing site. Values >1 (horizontal dotted line) indicate PSS.