Active genes are packaged into an altered nucleosome structure forming a chromosomal domain defined by increased sensitivity to nucleases. This structure, reflecting a potential for transcription, contains sites hypersensitive to nuclease digestion adjacent to the coding regions and may also be distinguished by specific non-histone proteins, variant or modified histones or modified DNA. Its formation, by unfolding of a tightly packed chromatin fibre by factors which might affect DNA supercoiling, may be the first step in gene activation.