A fission yeast cell cycle START gene has been identified, sct1. Loss of sct1 function results in cell cycle arrest at START and simultaneously in derepression of the mating pathway. sct1 therefore functions both as an essential activator of the mitotic cell cycle and as a repressor of differentiation. p72sct1 shares 36% sequence similarity with p85cdc10. p72sct1 is shown to act in partnership with p85cdc10 in a cell cycle regulatory transcription complex. A single dominant mutation within the putative DNA-binding domain of p72sct1 renders the cell independent of cdc10 function for the execution of START.