 The first mechanism proposes that, without dissociating from the enzyme complex, the primary site is phosphorylated and immediately shifted from the active site to the Cks1 binding pocket to allow for the additional phosphorylation of other CDK sites.The second mechanism proposes that the phosphorylated primary site binds to another location and is continuously bound while other CDK sites bind to the active site in a sequential manner for multi-phosphorylation.Simulations predict that the probability of a second phosphorylation event after the first, without dissociation, is 40% and 20-40% for the first and second mechanism, respectively.