New approaches to the mapping of chromosomal domains.

Although it is generally accepted that the chromosome is divided into elementary subunits, the structural and functional domains, the organisation of these structures at the molecular level is not well understood. In particular, the domain boundaries are not easily identifiable. Several possible candidates such as MARs/SARs, insulators, LCRs, palindromic sequences, or easily melting sequences have been found in the regions having properties one would except for boundaries. None of these elements, however, has been found in all of the constructs functioning as boundaries in tests in vivo. Recent work suggests that the common denominator might be the presence og GC-rich oligonucleotide stretches and the formation of the chromatin hypersensitive sites. A model is discussed in which "unusual" structures, in particular the four-stranded DNA sequence elements containing unpaired bases, play the role of domain boundaries.