The fission yeast Schizosaccharomyces pombe has been used to identify gene functions required for the cell to become committed to the mitotic cell cycle and to initiate the processes leading to chromosome replication in S-phase. Two gene functions cdc2 and cdc10 must be executed for the cell to traverse 'start' and proceed from G1 into S-phase. Before the completion of these two functions the cell is in an uncommitted state and can undergo alternative developmental fates such as conjugation. A third gene, suc1, has also been identified whose product may interact directly with that of cdc2 at 'start'. The molecular functions of the genes involved in the completion of 'start' have been investigated. The cdc2 gene has been shown to be a protein kinase, suggesting that phosphorylation may be involved in the control over the transition from G1 into S-phase. The biochemical functions of the cdc10 and suc1 gene products have not yet been elucidated. A control at 'start' has also been shown to exist in the budding yeast Saccharomyces cerevisiae. Traverse of 'start' requires the execution of the CDC28 gene function. The cdc2 and CDC28 gene products (lower-case letters represent genes of Schizosaccharomyces pombe, and capital letters genes of Saccharomyces cerevisiae) are functionally homologous, suggesting that the processes involved in traverse of 'start' are highly conserved. An analogous control may also exist in the G1 period of mammalian cells, suggesting that the 'start' control step, after which cells become committed to the mitotic cell cycle, may have been conserved through evolution.