On the basis of their primary structure, the lysine-rich histones are a unified family of proteins. Each has an amino acid chain which falls into three distinct domains. Only the central domain (approximately 80 residues) is in a folded conformation. It is protected from trypsin digestion in chromatin and corresponds to the segment of highest sequence conservation. Without the flanking domains it is able to close two full turns of DNA in the nucleosome and can thus locate the H1 molecule.