To investigate how different phosphorylation patterns affect the conformational properties of the CTD, we expanded our CTD kinase toolkit to include Dyrk1a (dual specificity tyrosine-phosphorylation-regulated kinase 1A), a kinase associated with Down syndrome that has been reported to phosphorylate Ser2 specifically in vitro (39, 40, 41). Here, DYRK1A is linked to Down syndrome.