Cell fusion is the process of fusion of the membranes of two or more cells and results in the formation of cells with multiple nuclei. It occurs at various stages of the natural development of organisms, such as in the first step of fertilization of an oocyte with sperm and in myotube formation by fusion of myoblasts during differentiation of skeletal muscles. Artificial cell fusion can be induced by the addition of a high concentration of Sendai virus, an enveloped virus of the paramyxovirus group, as was demonstrated in 1957-63 (1-3). This finding coincided with cell biology’s beginning focus on the culture of somatic cells of birds and mammals. In 1961, Barski et al. (4) reported the appearance of hybrid cells after a few months of mixed culture of two different mouse cancer cell lines. These hybrid cells had a single nucleus containing chromosomes from both parent cell lines, and their appearance was considered to be due to the spontaneous fusion of cells of the two cell lines.