Control over the mitotic cell division cycle of the fission yeast Schizosaccharomyces pombe is exerted at two main points, mitotic entry and an earlier point, situated prior to DNA replication, known as Start. At Start, cells become committed to one of a number of developmental pathways. The genes cdc2⁺ and cdc10⁺ are both required for commitment to the mitotic pathway. The cdc22⁺ gene encodes an enzyme which functions in S phase, and is periodically expressed at the G1/S boundary. Cdc10 binds elements in the cdc22⁺ promoter known as MCBs, and synthetic MCBs, inserted upstream of a reporter gene, confer periodicity of expression upon it. A second reporter gene has been constructed, under the control of sequences from the cdc22⁺ promoter, and this has been used, together with the MCB-controlled reporter, to investigate the regulation of cdc22⁺ under various conditions of cell cycle arrest. It has been shown that cdc2⁺ is not required at Start for MCB-mediated gene expression. In cdc10^ts arrested cells, cdc22⁺, and the reporter gene under the control of the cdc22⁺ promoter, are both expressed. In contrast, the reporter gene under only MCB control is not expressed independently of cdc10⁺. This suggests the existence of at least one cis-acting element within the cdc22⁺ promoter, additional to MCBs and required for cdc10⁺-independent gene expression. The roles of the two clusters of MCBs that are found within the cdc22⁺ promoter have also been investigated.