The cdc 2 (previously called wee 2) cell cycle start gene of Schizosaccharomyces pombe, which is required for start and the control of mitosis, has been isolated from an S. pombe gene bank by complementation of a cdc 2 mutation. A functionally homologous sequence which complements the cdc 2 mutation has also been isolated from a Saccharomyces cerevisiae gene bank and this sequence has been shown to contain the cdc 28 cell cycle start gene of S. cerevisiae. It is concluded that the cdc 2 and cdc 28 genes perform homologous cell cycle control functions in the two organisms.