Mammalian DYRKs are a subfamily of mitogen-activated protein kinase-related protein kinases and are originally discovered on the basis of homology to the Saccharomyces cerevisiae Yak1 and Drosophila mini-brain kinases. DYRKs possess Ser/Thr phosphorylation activity as well as autophosphorylation activity on Tyr residue(s). Two isoforms of DYRK, DYRK1A and DYRK1B, co-immunoprecipitate with HAN11 when coexpressed in COS cells indicating that the proteins interact in mammalian cells. Co-expression of DYRK1A, DYRK1B, or DYRK2 with a series of glycogen synthase mutants with Ser/Ala substitutions at the phosphorylation sites in COS cells revealed that protein kinases cause phosphorylation of site 3a in glycogen synthase. Control of glycogen synthase by DYRK represents a novel mechanism, and a potentially novel pathway, for the regulation of glycogen synthesis.