During development, Polycomb group (PcG) proteins maintain patterns of gene repression that are mediated by homeobox-containing proteins. These PcG chromatin-binding proteins form several multicomponent complexes, but how these repress gene transcription in vivo is unknown. Ficz and colleagues now report that PcG protein complexes exchange rapidly, most within 2 minutes, in living Drosophila embryos, which suggests that PcG repression occurs through dynamic competition with other chromatin-binding proteins (see p. 3963). By using fluorescence recovery after photobleaching (FRAP) microscopy, the researchers determined the kinetic properties of two PcG-GFP fusion proteins in whole Drosophila embryos, wing imaginal discs and salivary glands. They show that PcG complexes are rapidly exchanged throughout development and that complexes at different chromosomal sites have different exchange rates. Thus,PcG complexes maintain the long-term repression of developmental regulatory genes dynamically rather than by statically limiting the access of transcriptional activators to chromatin.