Sic1 inhibits the activity of Cdc28.Clb5 complexes in late G1, creating a delay between cell cycle commitment and S phase initiation. The ultimate purpose of this delay is unknown but loss of Sic1 activity negatively affects genomic stability and cellular viability. Sic1 levels are controlled by periodic changes in transcription rates and protein stability. The latter control is mediated through the Cdc34 ubiquitin transferase and, possibly, Cdc28.Cln activity. Possible roles of Sic1 in the G1/S and the M/G1 transitions are discussed.