The copy-DNA was synthesized on the mRNA fraction which was isolated from MOPC 21 mice myeloma by reverse transcription. This copy was completed with another chain without adding the exogenous primer, with the aid of the Klenov fragment of DNA polymerase I. After treatment of the double-stranded pin DNA with endonuclease S1, the poly(dA) sequences were built up using terminal deoxynucleotidyl transferase. The building up of the poly(dT) sequences at the DNA 3'-termini of pBR322 plasmid previously treated with BamHI was performed in a similar way. The average length of connecting polynucleotide sequences was 50 nucleotides. After annealing and transformation of the cells with Escherichia coli hybrid plasmids, selection of clones was made for ApRTcS phenotype. Further, we applied a stepwise selection of clones by means of increasing the specificity: colony hybridization, quantitative hybridization of the excess of plasmid DNA with (32P)-cDNA and hybridization of plasmid DNAs with mRNA units which were transferred from electrophoregrams to the diazo-papers. As a result, bacterial clones were selected which contained gene fragments showing the ability to hybridize with the RNA fraction characterized by mRNA mobility of the light-chain immunoglobulin G.

In the course of exposures to the immunological agents (BCG, complete Freund's adjuvant) during the postnatal period, the resistance to tumor development induced with chemical carcinogens was found to increase in C57BL/6 mice, particularly in BALB/c and random-bred ones; meanwhile in C3H/He mice it declined. At the same time C3H/He mice showed a decrease in the resistance to the development of spontaneous tumors of the mammary glands. The changes recorded correlated with those seen in the immunocompetent organs.

Hamster embryo cells spontaneously transformed in vitro (HETr) with low metastatic activity were not capable to depress natural antitumor resistance in vivo in contrast to some other lines of Syrian hamster tumor cells. The resistance, depressing and metastatic activities of HETr parental cells and their variants obtained from lung metastases were studied and compared. The direct correlation was found between the resistance-depressing and metastatic activities of the cell variants studied. Five of eight cell variants obtained from lung metastases ocf HETr were capable to depress natural host resistance to the growth of transplanted tumor cells. They were the most active variants in the lung colonization test. It is suggested that in the course of metastasizing there takes place the selection of the tumor cell variants capable of depressing natural antitumor host resistance.

Necessity of oncogene for the neoplastic cell transformation is substantiated. The oncogene may be introduced into the cell by a virus; be present in the cell from its formation but be repressed by products of regulatory genes; be arisen from separated DNA segments. The latter possibility is discussed in view of data on the presence of mobile elements and Sarc sequences in the cell genome.

Alterations in ploidy of hepatocyte nuclei were followed in experiments on 190 rats with transplantable tumors during liver regeneration and x-ray irradiation. The presence of the growing transplantable tumor in the animal's body led to alterations in the nuclear structure of hepatocytes, manifesting in the increased ploidy. X-Ray irradiation brought about a rise in hepatocyte ploidy which was more remarkable the higher the dose. Under such conditions, polyploidy of hepatocyte nuclei was more pronounced in tumor-bearing rats than in those without tumors. An abrupt increase in ploidy of hepatocyte nuclei occurred in the regenerating liver of tumor-bearing animals both non-irradiated and, particularly, irradiated ones.