Recently it has been found that B-type cyclins in fission yeast regulate the activation of the cdc2 kinase to promote the onset of both DNA replication and mitosis. cig2 is the major G1 cyclin while cdc13 is the principal mitotic cyclin. cdc13 also has an additional function in G2 phase, preventing more than one round of DNA replication per cell cycle. In opposition to these cyclins the rum1 inhibitor, a protein present exclusively in G1, prevents premature activation of the cdc2/cig2 and the cdc2/cdc13 complexes until cells have reached the critical cell size required to pass Start and initiate a new cell cycle.