Experimental Cell Research 65 (1971) 140-144 CELL CYCLE OF MULT1NUCLEATE CELLS AFTER CELL FUSION A. WESTERVELD and MARIA A. FREEKE Medical Biological Laboratory of the National Defence Research Organization TNO, Rijswijk (ZH), The Netherlands SUMMARY The cell cycle of binucleate and nmltinucleate cells was studied after fusion of synchronized cells. A prolongation of the S phase in the nuclei of the binucleate cells was observed. Mitosis was not always synchronized in the homokaryons. The duration of the G1 phase in binucleate S/G1 cells is shortened. The binucleate cells and multinucleate cells are able to progress through the cell cycle for more than one generation. Okada et al. [1] were able to induce the formation of multinucleate cells by fusing together single cells in the presence of Sen- dai virus. Harris & Watkins [2], Harris [3] and Harris et al. [4] showed that both dif- ferentiated and undifferentiated cells from different species could successfully be fused together to form viable heterokaryons. In these multinucleate cells DNA synthesis has been extensively studied. In heterokaryons formed by the fusion of Ehrlich and HeLa cells the nuclei do not synthesize their DNA synchronously [2]. Johnson & Harris [5] observed in these, HeLa-Ehrlich heterokaryons even a form of antisynchrony. Most of the Ehrlich nuclei synthesized DNA whilst most of the HeLa nuclei did not. In contrast to these observations with heterokaryons it was found that when homo- karyons (cells containing genetically similar nuclei) are fused together, DNA-synthesis in the first cell-cycle after cell fusion is syn- chronized [6, 7]. Exptl Cell Res 65 Recently Rao & Johnson [8] showed an induction of DNA-synthesis in nuclei of multinucleate HeLa cells produced by fusion between cells in different phases of the cell cycle. In the heterophasic G1/S cells DNA- synthesis was induced in more than 50 % of the GI nuclei within 2 h after cell fusion. DNA-synthesis was not induced in G2 nu- clei after fusion of S phase cells with G2 phase cells. In the present study DNA synthesis and the duration of the phases of the cell cycle during several cell generations after hybrid- ization of genetically identical cells was in- vestigated. MATERIALS AND METHODS A heteroploid cell line of human kidney origin (T cell) [9] was used in the experiments. These cells have a cell cycle of 24 h (G1, 12 h; S, 8 h; G2, 4 h) [10]. The cells were cultivated in F12 medium without thymidine, supplemented with 6% newborn calf serum and antibiotics. The cells were synchronized with the excess thymidine method [11, 12]. The technique used for inducing cell fusion was essentially similar to that described by Okada [13].