In the fission yeast Schizosaccharomyces pombe Mik1p, in combination with Wee1p, is an important inhibitor of mitosis through direct phosphorylation of Cdc2p. Here we present the observation that mik1(+) is transcribed during G1- and S-phase in normally dividing cells. mik1(+) transcription is regulated by the MCB-DSC1 system, which controls expression of other genes at the G1-S interval. mik1(+) is shown to be an important target of MCB-DSC1 as it is epistatic for the mitotic delay phenotype displayed in cdc10-C4 cells, which are mutated in a component of DSC1. The mitotic delay in cdc10-C4 cells is bypassed by cdc2-1w, suggesting that mik1(+) acts directly on cdc2(+), with no checkpoint function involved. Thus, mik1(+) represents a new type of MCB-DSC1 regulated gene in fission yeast, whose gene product is exclusively expressed during G1- and S-phase to prevent premature mitosis during this cell cycle stage.