Abstract: The present invention relates to a method for determining sensitivity to a cyclin-dependent kinase 4/6 (CDK4/6) inhibitor in a human subject having breast cancer, the method comprising determining in a sample from the subject the level of expression of 11 genes as defined in the claims. The present invention further provides kits of parts, nucleic acid arrays, and uses thereof for determining sensitivity to a CDK4/6 inhibitor in a human subject having breast cancer. The present invention also relates to methods of treating a human subject having breast cancer, comprising administering a therapeutically effective amount of the CDK4/6 inhibitor to the subject.

Abstract: 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.