Résumé : The present invention concerns an in vitro assay for determining the activation status of endogenous CDK4 in eukaryotic cells, said assay comprising the steps of: providing eukaryotic cells maintained in a quiescent state, said eukaryotic cells comprising a cyclin D/CDK4 fusion protein, wherein the CDK4 part of the fusion protein is present in a hypophosphorylated form; inducing proliferation of said eukaryotic cells; isolating the cyclin D/CDK4 fusion protein from said eukaryotic cells; and measuring the activation status of said isolated cyclin D/CDK4 fusion protein, thereby determining the activation stats of endogenous CDK4 in said eukaryotic cells.