Abstract: A novel method of immortalizing cell lines, as well as cell lines immortalized by the method. According to the method of the present invention, a gene which confers on a cell the ability to grow in tissue culture is introduced, using known techniques, into such a cell. This gene, referred to as a growth promoting gene, is under control such that when the gene action or function is turned down or off, the cell in which it resides switches to a new differentiated state. In one embodiment, the temperature sensitive (ts) form of the oncogene derived from the simian virus SV40 is the growth-promoting gene.