1. A method for producing stable cell lines of mammalian neural precursor cells in vitro, comprising the steps of: a) preparing a culture of neural precursor cells in a serum-free medium; b) culturing the neural precursor cells in the presence of a first mitogen, wherein said first mitogen is selected from the group consisting of aFGF, bFGF, EGF, TGFα and combinations thereof; c) contacting the cells with an agent capable of being taken up by the cells and capable of expressing a c-myc gene; and d) further culturing the cells in a medium containing the first mitogen and a second mitogen, wherein said second mitogen is selected from the group consisting of aFGF, bFGF, EGF, TGFα serum and combinations thereof, with the proviso that the second mitogen is other than the first mitogen.