Malignant tumors are composed of cells that have lost their proliferation control and multiply independently of a physiologic need for increase in cell number in the tissue. Cells that have lost these proliferative controls are often referred to as “transformed.” Transformation of a mammalian cell has been defined as the acquisition of permanent disturbances of growth and/or locomotion control (1 ). This definition of transformation implies a phenotypic change that has been found to be induced either by the introduction of a new gene into a cell or by the mutation of the genome through either irradiation or deoxyribonucleic acid (DNA)-damaging agents. Genes that can be introduced into normal cells to produce a transformed phenotype are usually derived from oncogenic viruses. Thus the definition of transformation in mammalian cells differs form the definition of transformation in bacteria where the term refers to the introduction of any new gene into the bacterial genome.
