How Polycomb group complexes are targeted to specific genes remains a challenging question. Research in Drosophila melanogaster has shown that specialized Polycomb response elements (PREs) are key for targeting of Drosophila Polycomb complexes. PREs are necessary and sufficient to recruit all Polycomb complexes but they also recruit proteins that counteract Polycomb repression in cells where target genes have to remain active. The ability to simultaneously recruit repressors and antirepressors makes PREs switchable cellular memory modules. PREs comprise flexible combinations of recognition sequences for multiple sequence-specific DNA-binding proteins. Polycomb complexes cannot bind DNA on their own so these adaptor proteins mediate the recruitment. Most of the PRE-bound adaptor proteins interact with Polycomb complexes weakly. However, their individually weak interactions combine to produce robust recruitment. Such combinatorial recruitment is key for the epigenetic property of the Polycomb system to repress target genes only if they have been repressed during previous cell cycles.