Main . Polycomb group (PcG) proteins form complexes that interact with chromatin to repress transcription1 , 2 , 3 , 4 , 5 , 6. Polycomb-dependent 3D-structural changes have been observed at the kilobase-to-megabase scales by fluorescence microscopy7 , 8 , 9 , 10 , 11 , 12 , 13 , 14 , 15 , 16and Hi-C14 , 17 , 18and the nucleosome scale by in vitro electron microscopy19 , 20. Changes at both these scales have been described as ‘com...