Mechanisms of tumor progression.

tory, although the time course may be quite variable. This phenomenon has been termed tumor progression, and Foulds (1,2) first pointed out that the process appears to develop in a stepwise fashion through qualitatively different stages. Others, including the author, have subsequently suggested that this biological and clinical progression might reflect, at least in part, the sequential appearance within the tumor of increasingly genetically altered subpopulations with new characteristics (35). It is the purpose of this brief discussion to summarize some current thoughts concerning tumor progression and suggest where present investigations may be leading.