It is now well established that the entire genetic information is con­ served in DNA base s equence. Each cell of the multicellular organism con­ tains the same DNA. In other words, each cell possesses the same genetic information. However, due to the pronounced differences in their protein composition, these cells are quite different in their properties. Thus only part of the genetic information is realized in each cell. The realization of genetic information, or gene expression, consists of a number of steps. The more important steps are: transcription or RNA synthesis, transport of RNA to the sites of protein synthesis, and translation or protein synthesis in ribosomes on the messenger RNA template. At any step of this complex process, some limitation of information may occur and the "information stream" may narrow. As a result, only part of the DNA base sequence is realized in polypeptide chains. The first step of such a limitation process is transcription. A number of papers have appeared to indicate that RNA in different tissues of the same organism is synthesized on different parts of the genome. This has been demonstrated with the aid of the competitive-hy­ bridization technique between DNA and RNA's isolated from different tis­ sues of the same species (1). Thus the question of the mechanism of this limitation of transcription arises. What are the molecular mechanisms of this "switching off" of some DNA molecules from the biosynthesis of messenger RNA (or their "mask­ ing") ? In 1945 the English scientists Stedman & Stedman (2) developed the hypothesis that histones, the bas ic proteins of the cell nuclei, inhibit gene activity in the cell. It should be pointed out that the appearance of histones in evolution takes place at the same time that organisms acquire two other fundamental properties . One of these properties is the separation of the nu­ cleus from the cytoplasm by a membrane. This distinguishes two main groups of organisms : prokaryotes and eukaryotes. Prokaryotes do not con­ tain histones in the nuclei [see for an example (3)]. The second fundamental property, which is especially pronounced in multicellular organisms, is differentiation. Thus the appearance of histones in evolution roughly coincides with the appearance o f a well-defined nucleus (and chromosomes ) and with differentiation. Therefore, one can assume histones to be involved in the organization of chromosomes and in prolonged switching off of genes. This long-term masking of some of the genes may