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Many Growth Factors May Not Be Growth Factors

Department of Pathology, SM-30, University of Washington, Seattle, Washington 98195

Both organ growth and tumor growth are dependent upon a biased ratio of cell births to cell deaths; this ratio is independent of the frequency of mitosis. Thus, so-called growth factors that affect only the frequency of mitosis are not really growth factors. I shall advance two hypotheses: that the normal adult ratio of cell births to cell deaths is maintained by the activity of a factor or factors produced by the stem cells; and that the differentiating cells, as well as the stem cells, produce a different factor that limits the frequency of mitosis.

¹ To whom requests for reprints should be addressed, at 407 Lake Avenue West, Kirkland, WA 98033.

Received 8/19/91. Accepted 11/11/91.