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Chromatin conformation and dynamics is central to gene functions including packaging, regulation, and repair. At the molecular level, the basic building block of chromatin is a nucleosome core particle (NCP) made of∼ 147 base pairs (bp) of dsDNA wrapped around an octamer of histone proteins. These NCPs are connected by short 20-80 bps of linker DNA as beads on a string. Key factors determining the packaging of NCP arrays to form chromatin include ionic condition, linker DNA length, and epigenetic modification, especially of the histone tails. We have investigated how the conformations of model tetra-NCP arrays are modulated by these factors using small angle x-ray and neutron scattering. Here we present recent studies of the effects of ion (KCl, up to 200 mM) and histone modification (tail deletions) on NCP arrays. Our combined x-ray and neutron scattering measurement makes it possible to learn about …