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The paper deals with the analysis of literary data on the tumor cell heterogeneity. Phenotypic, genetic and epigenetic mechanisms of heterogeneity are considered. The heterogeneity of metastasis is considered too. The importance for the biology of populations of tumor cells and the sensitivity of tumors to therapeutic treatment are discussed.

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The methods of hybridization in solution and blot hybridization showed that spleen cells from BALB/c mice contain "silent" genes which can amplify and change their structure after infection by Rauscher leukaemia virus. The "silent" gene product is nuclear 35S RNA detectable by comparative electrophoretic analysis of the heterogeneous nuclear RNA from leukaemic and normal cells. About 7% of this 35S RNA is represented by the virus-specific sequences, but a major part is represented by the cellular sequences. In order to study the expression of the sequences homologous to 35S RNA in leukaemic and normal cells, hybridization in solution was used. Expression of the complete copies of 35S RNA was observed in nuclei of virus-infected cells, whereas this RNA in the cytoplasm is represented by the incomplete copies. The expression of the sequences homologous to this 35S RNA in normal mouse spleen cells was not revealed.

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Modern investigations of structural and functional organization of oncoviruses are analyzed. The data available in the literature on oncovirus interaction with host cell membrane (adsorption, penetration by endocytosis and fusion, uncoating) as well as the cell and viral receptors are elucidated. The results of the studies on retroviral organization of genome, its integration with cellular genome, transcription and translation are given. The functions of long terminal repeats are described. The characteristic of the end step of life cycle of retroviruses (assembly, budding and maturation) is presented.

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The methods of hybridization in solution and blot hybridization have shown that the BALB/c mice spleen cells contain "silent" genes which can amplificate and change their structure following the infection by the Rauscher leukemia virus (RLV). The product of these gene activation is nuclear 35S RNA detectable by the comparative electrophoretic analysis of heterogeneous nuclear RNA of leukemic and normal cells. The expression of complete copies of 35S RNA was observed in nuclei of RLV-infected cells, while in cytoplasm this RNA is represented by incomplete copies. The expression of the sequences homologous to this 35 S RNA in normal mice spleen cells was not detected.

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The paper is concerned with composition of neutral lipids and phospholipids, the regularity of lipid bilayer and structural reorganization of plasma membranes, and membranes of smooth and rough cell reticulum of thymus and Pliss lymphosarcoma are studied at linear and stationary growth phase. No qualitative differences are found in the fatty-acid composition of lipid membranes in normal and tumour cells. In plasma membranes of phospholipids and in membranes of smooth reticulum of tumour cells the unsaturated lipid component increases in the process of growth, the cholesterin/phospholipids ratio decreases, fluidity of the lipid bilayer diminishes and structural heterogeneity of these membranes rises while in membranes of rough reticulum the saturation of lipids increases, but the cholesterin/phospholipids ratio does not change. The temperatures of structural reorganization also does not change, which evidences for a less structural lability of membranes of rough reticulum as compared with other membranes.

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A technique of molecular weight estimation in proteins according to their electrophoretic mobility in the gradient polyacrylamide gel is used. The method permits analysing the highly heterogenic protein mixtures and determining the molecular weight of proteins within the range of 5000--400.000 simultaneously. The data of the protein spectrum analysis of the liver cell nuclei and some tumors are presented.

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Cells of rat Thurzo-Svee leukemia produce C-type virus. Viral particles are shown to contain high-molecular weight RNA and RNA-dependent DNA polymerase the virus possesses mammalian gs-3 antigen and rat species-specific gs-1 antigen and thus appears to be of rat origin.

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The sedimentation coefficient, molecular weight and the type of nucleic acid of Aedes cantans iridescence virus were determined, and its proteins were analysed in a dissociating system by electrophoresis in polyacylamide gel. Ultrathin sections of affected larvae were examined.

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The structural proteins of virus-like particles isolated from ascitic fluid of rats with experimental Shvec erythroleukosis were studied by polyacrylamide gel electrophoresis. The bulk of protein (60-70%) is divided into five major bands corresponding to a molecular weight of 15,000, 20,000, 34,000, 48,000 and 61,000 daltons. Besides these bands, some distinct minor ones corresponding to a molecular weight ranging from 41,000 to 82,000 daltons were always present. There were also some slight bands that are not developed in scanning on the densitometer. These bands occurred in the region corresponding to a molecular weight above 50,000 daltons. The gels stained to detect polysaccharides revealed a distinct red colour band of a molecular weight 80,000 daltons and 3 slight of a lower molecular weight which are probably the degradating products of the main glycoprotein component. The electrophoretic pattern of the 1.16 g/cm3 fraction obtained from the healthy rat liver and Ehrlich ascitic fluid differs from the electrophoregram of the virus-like particles isolated from the rats with Shvec erythroleukosis.

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