Coordination between Cell Growth and Cell Cycle Transit in Animal Cells

Most studies of the control of animal cell prolifer-ation have been performed in various model systems in vitro, in which cell proliferation can be modulated in a controlled fashion. Although each in vitro system has its own particular features and limitations, and al-though it is unclear to what extent in vitro data can be extrapolated to the in vivo situation, some general features of proliferation control of animal cells have emerged from the in vitro studies. Normal cells usually cease to proliferate in a cell-cycle-specific way. They arrest in G 1 or enter a state of quiescence (Go) from G 1 after depletion of serum or