Polycomb response elements (PREs) show both site-specific and inherent behavior. The Fab-7 (blue) and vg (purple) PREs are shown at two different landing sites (A,B). Each PRE has inherent properties in repressing the mw reporter (blue and purple arrows). We propose that each PRE also interacts specifically with different enhancers or repressors at each landing site (green and yellow arrows; enhancers/repressors shown as solid symbols). The output of the mw promoter depends on the properties of the PRE, on its interaction with the landing-site environment, and on whether the PRE is heterozygous or homozygous (see main text for details).