To study possible involvement of polypurine and polypyrimidine DNA tracts capable of forming triple-stranded structures (the H-form of DNA) in compaction of eukaryotic chromosomes, an in silico search for complementary polypurine and polypyrimidine sequences was carried out within 12 eukaryotic genes. It was shown that, in chromosomal gene loci, 10-11 bp polypurine and polypyrimidine tracts potentially capable of interacting with each other with the formation of triplex structures ("structuring" regions) are located in predominantly in introns and gene-flanking regions. In vivo, such DNA-DNA interactions can result in the chromosomal gene domain folding into several small loops. The character of the DNA triplex-mediated compaction of chromosomal gene loci may be related to gene functioning. A similar analysis of long (LINE) and short (SINE) interspersed repeat sequences, as well as of satellite DNA, showed essential resemblance between the compaction mechanisms of coding and noncoding chromosome regions.