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The S-phase-promoting factor (SPF) is inhibited by Sic1 (a protein which also is a stoichiometric inhibitor of Cdk1-Clb and B-type cyclins). Sic1 releases SPF from inhibition after it is phosphorylated by G1-cyclin-CDK. The phosphorylation marking SPF for polyubiquitination by the SCF ubiquitin ligase. Then the active SPF trigger initiation of DNA synthesis.