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In eukaryotes,the packaging of genomic DNA into chromatin plays a critical role in gene regulation.However,the dynamic organization of chromatin fibers and its regulatory mechanisms remain poorly understood.Using single-molecule force spectroscopy,we reveal that the tetranucleosomes-on-astring appears as a stable secondary structure during hierarchical organization of chromatin fibers.Thestability of the tetranucleosomal unit is attenuated by histone chaperone FACT (Facilitates Chromatin Transcription) in vitro.