[en] This paper reports that during either chemical or radiation carcinogenesis, the progression of target stem cells through a premalignant to a malignant state is accompanied by a variety of biochemical and morphological alterations. Presumably these phenotypic alterations result from changes in the levels of expression of cellular genes or qualitative changes in the encoded gene products. A class of cellular genes that are known to be altered during multi-stage carcinogenesis and are thought to play a functional role in tumor progression are the cellular proto-oncogenes. These genes encode for proteins that play important roles in the control of growth and differentiation and could be altered and activated by environmental carcinogens. Activation of these proto-oncogenes involves point mutations, deletions, translocations and amplification. These types of genetic alterations are known to be induced by environmental mutagens and carcinogens. In addition to the proto-oncogenes, there is a class of tumor suppressor genes whose inactivation may be required for the expression of the tumorigenic phenotype because the genes' active expression may override the transforming action of highly expressed oncoproteins