 In the case of beta-catenin, the most important priming kinase is Casein Kinase I (CKI).Once a serin-threonine rich substrate has been "primed", GSK3 can "walk" across it from C-terminal to N-terminal direction, phosphorylating every 4th serine or threonine residues in a row.This process will result in dual phosphorylation of the aforementioned β-TrCP recognition motif as well.