 At the START point in the yeast cell cycle, the G1-cyclins Cln3, Cln1 and Cln 2 activate Cdc28.The activated complex will phosphorylate Sic1 at multiple sites which leads to its degradation by the SCF complex.When Sic1 is degraded, the Cdc28-Clb complex is no longer inhibited and the cell can enter the S/M-phase.