Regulator of cell cycle G1 phase progression. Ensures the correct sequence of S phase and mitosis in the cell by acting as an inhibitor of the cdc2 mitotic kinase. Probably interacts with cdc2 to inhibit its action until the cell mass for Start is reached. Determines the length of the pre-Start G1 period and prevents mitosis from happening in early G1 cells. Required for maintaining pheromone-induced G1 arrest. Acts as an adapter protein since interaction with cdc13 promotes cyclin proteolysis during G1. Becomes a target for degradation at the G1/S phase transition, following phosphorylation by cig1-associated cdc2 at the G1/S phase transition. Interacts with cdc13, cig2 and pop1. Expression increases in nitrogen deprived environment, which is important to cause delay of the G1 phase of the cell cycle in response to nitrogen starvation. Phosphorylated by cig1-associated cdc2 which leads to increased stability. Phosphorylation by MAPK reduces cdc2 kinase inhibitor ability.