The alteration of genetic expression ubiquitously seen in both preneoplastic and neoplastic tissues has been investigated for many years in the hope that the critical molecular changes resulting in cancer can be elucidated. The alteration of the expression of specific genes has already been employed in diagnostic and even screening procedures for this disease. In the past many observations of such alterations have led to a variety of theories but not definitive generalizations. Studies of the alteration of genetic expression may now be viewed in the light of our understanding of the multistage nature of neoplastic development. This brief review describes a number of genes the expressions of which are altered during the stages of initiation and promotion, in contrast to the alteration of expression of genes during the stage of progression. The promotion stage is concerned primarily with the chronic interaction of promoting agents in the environment with the genetic apparatus of the cell, played out on the altered genetic background resulting from the stage of initiation. In contrast, the progression stage is characterized primarily by an evolving karyotypic instability resulting in continual genetic changes during this stage. On the basis of these distinctions it is possible to identify genes the altered expression of which is unique to the stage of progression. The identification of these genes and an understanding of mechanisms resulting in their altered expression will allow not only a better molecular characterization of the progression stage but also the quantitative analysis of neoplastic development in several model animal systems as well as eventually in the human.