Histone-DNA contacts within a nucleosome influence the function of trans-acting factors and the molecular machines required to activate the transcription process. The internal architecture of a positioned nucleosome has now been probed with the use of photoactivatable cross-linking reagents to determine the placement of histones along the DNA molecule. A model for the nucleosome is proposed in which the winged-helix domain of the linker histone is asymmetrically located inside the gyres of DNA that also wrap around the core histones. This domain extends the path of the protein superhelix to one side of the core particle.