Mutations in the yeast gene ARD1 lead to inability to respond to alpha-factor, inability to enter stationary phase, and inability to sporulate, suggesting an important role for the ARD1 gene product in controlling the switch between the mitotic cell cycle and alternative cell fates. MATa, ard1 cells seem to be defective in the expression of all a-specific functions, whereas MAT alpha, ard1 cells respond normally to a-factor. We propose that ARD1 is required for the expression of genes involved in a-mating functions, stationary phase, and sporulation. The ARD1 gene has been cloned and sequenced; there is weak homology between the C terminus of the ARD1 protein, the C-terminal region of MAT alpha 2, and the homeo box.