 Thus, in Sic1 mutants with one Cdk1 phosphorylation site or only phosphoserines present, Cks1 is unable to properly bind to the substrate and promote Sic1 multi-phosphorylation.This provides a strong argument for a processive phosphorylation mechanism instead of the previous theory of a random distributive phosphorylation model.In addition to requiring threonine, Cks1 binding to Sic1 can be enhanced with the introduction of a proline residue at the -2 position relative to the threonine residue.