On deciding which factors regulate cell growth

Many conditions affect the growth of animal cells in culture. Some, such as essential nutrients, are necessary for growth as precursors of macromolecules. Other substances appear to have regulatory functions. Their presence or absence determines whether the cell will continue to grow or move into a resting, quiescent state. In particular certain factors (usually provided by serum) are thought to have regulatory roles. To decide which factors are regulatory is a difficult problem. We propose here that no single criterion is sufficient to permit a decision and that the best course at present is to apply several criteria. Three such criteria are proposed. To the degree that the tests are satisfied we can tentatively decide whether or not a given factor is regulatory.