The mechanisms responsible for correct timing of DNA synthesis within the cell cycle and for limiting replication to one round per cell cycle are basically similar in the two model yeasts, Saccharomyces cerevisiae and Schizosaccharomyces pombe, despite many differences in detail. In both cases, the timing of initiation and the prevention of additional rounds are controlled by the activity levels of B-type cyclins. These similarities are likely to extend to other eukaryotic organisms.