Populations of non-proliferating cells are often referred to as being in a quiescent, or G0, state. For unicellular microorganisms this quiescent state is a regulated response to a limitation of the nutrient supply, whereas for multicellular organisms, quiescence can also be induced by hormones, growth factors or contact with other cells. Indeed, most cells, including those in multicellular organisms, are found in a quiescent state with respect to cell proliferation. Nevertheless, we understand relatively little about the quiescent state of eukaryotic cells. Genetic and molecular approaches with the unicellular budding yeast Saccharomyces cerevisiae offer an opportunity to investigate this important aspect of eukaryotic biology.

The budding yeast S. cerevisiae is widely used to investigate fundamental cellular activities, for two important reasons. First is the genetic and molecular facility that the yeast system provides. For example, classical (Mendelian) genetic analysis and mutant isolation are straightforward with this organism, and many strategies are available for gene cloning. Moreover, yeast offers the ability to easily and routinely transfer cloned genes back into the genome to replace the resident chromosomal gene in the proper chromosomal context, an approach that allows elucidation of gene-product function in an in vivo context (reviewed in ref. 9). Furthermore, the fact that the entire yeast genome, and thus the complement of yeast proteins, is known (8) provides a comprehensive perspective for functional studies. The yeast system has therefore been invaluable for the application of molecular approaches to the study of eukaryotic gene function.