Eukaryotic cell cycle progression during meiosis and mitosis is extensively regulated by reversible protein phosphorylation. Many cell surface receptors for mitogens are ligand-stimulated protein-tyrosine kinases that control the activation of a network of cytoplasmic and nuclear protein-serine (threonine) kinases. Over 30 plasma membrane associated protein-tyrosine kinases are encoded by proto-oncogenes, i.e., genes that have the potential to facilitate cancer when disregulated. Proteins such as ribosomal protein S6, microtubule-associated protein-2, myelin basic protein, and casein have been used to detect intracellular protein-serine (threonine) kinases that are activated further downstream in growth factor signalling transduction cascades. Genetic analysis of yeast cell division control (cdc) mutants has revealed another 20 or so protein-serine (threonine) kinases. One of these, specified by the cdc-2 gene in Schizosaccharomyces pombe, has homologs that are stimulated during M phase in maturing sea star and frog oocytes and mammalian somatic cells. Furthermore, during meiotic maturation in these echinoderm and amphibian oocytes, this is followed by activation of many of the same protein-serine (threonine) kinases that are stimulated when quiescent mammalian somatic cells are prompted with mitogens to traverse from G0 to G1 phase. These findings imply that a similar protein kinase cascade may oversee progression at multiple points in the cell cycle.