We have mapped the DNase I-hypersensitive sites (HSs) in Yeast Artificial Chromosomes (YACs) containing segments of human chromosomal DNA. One of the five HSs found in a YAC carrying the beta-globin gene cluster has been localised in the region, termed HS2, that is DNase I hypersensitive in most human cells. We have also identified a class of HSs in YACs containing DNA from the q11.2 band of human chromosome 21, which are located close to, or within, segments of the chromosome that are sensitive to restriction enzymes recognizing CGCG tetranucleotides.