We have cloned and sequenced the cdc13+ gene from fission yeast. When a major part of the cdc13+ gene is deleted from the chromosome, cells arrest in interphase, but partial loss of gene activity leads to cells containing condensed chromosomes, aberrant septa and a microtubular cytoskeleton with characteristics of both G2 and M. Expression of this phenotype is influenced by the nutritional status of the cell. Our results suggest that the cdc13+ gene function is required for the control of the G2 to M transition. It appears to play a role in regulating the separate pathways of events involved in the physical process of mitosis, for example in the reorganization of the cytoskeleton on transition from G2 to mitosis. The cdc13+ gene function interacts closely with both the yeast and human homologues of cdc2+, suggesting that mammalian cells may contain a cdc13+ homologue. The gene encodes a putative polypeptide of 482 amino acids, and a central region of 176 amino acids of this polypeptide is 50% identical with sea urchin cyclin. Therefore, the cdc13+ protein is cyclin related and could act as a regulator or substrate of the p34cdc2 protein kinase, which initiates mitosis.