The study of cultured human tumor cells is a most obvious approach in experimental human cancer research. For many techniques in virology, immunology, biochemistry, and biophysics, for example, large amounts of cells may be required and such quantities are usually provided only when the cultures develop into established cell lines; when this happens, thorough characterization also becomes possible. The development of cell lines, therefore, is of prime importance. Recent major advances in research with animal cell systems see m to be a prologue for present and future efforts directed toward...