In the nucleus of eukaryotic cells, DNA is packaged into a nucleoprotein complex known as chromatin (1 ). This complex provides the compaction and structural organization of the DNA for processes such as replication, transcription, recombination, and repair. The highly compact structure of chromatin does not only restrict the access of DNA to enzymes and regulating factors, it can even act as an activating principle bringing distant DNA-protein binding sites into close proximity.