The Yang and Yin of cell proliferation: an overview.

Cell proliferation is a regulated process. The major events that determine production of new cells, from growth factors stimulation to cell division, are briefly summarized. Regulation of cell growth is exerted on specific cell cycle events. It depends upon specially evolved biochemical mechanisms, based on molecules additional to the functional molecules which control catalysis, or genetic sequences which code for proteins (as for regulation of bacterial enzyme and gene activities). The principle of Yang and Yin--the balance between opposing natural forces--is emphasized as a fundamental property of growth regulation. Such a balance creates a dynamic state that is far more responsive to effectors than is a process modified by a unidirectionally controlling agent. Derangements of dynamically controlled processes can seriously upset metabolic balances in a cell. Our understanding of growth control and its derangements in cancer depends on discovery of these regulatory molecules and processes.