F. In vitro kinase assay with either wild type ATOH1 (WT) or non-phosphorylated form of the Serine-to-Alanine 309 (S309A). WT or S309A were incubated with CCND1 and CDK4 (4D) in the presence of [γ-32P]-ATP. While WT with CCND1 and CDK4 gave rise to strong signals (lanes 1,2 white arrowhead), S309A showed no obvious signal (dashed arrowhead indicated the predicted size of ATOH1). 