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 the steps of: - determining in a sample from the subject the level of expression of 11 genes or fragments thereof consisting of cyclin-dependent kinase inhibitor 2A (CDKN2A); cyclin E1 (CCNE1); transgelin 2 (TAGLN2); translocase of inner mitochondrial membrane 17 homolog A (yeast) (TIMM17A); RAB31, member RAS oncogene family; spindle apparatus coiled-coil protein 1 (SPDL1); nucleoporin 155kDa; F-box and leucine-rich repeat protein 5 (FBXL5); gelsolin; TP53 target 1 (non-protein coding) (TP53TG1); and protein phosphatase 1 regulatory subunit 3C (PPP1R3C); - providing three reference profiles of the level of expression of the 11 genes or fragments thereof: a first reference profile representing insensitivity to the CDK4/6 inhibitor; a second reference profile representing therapeutic sensitivity to the CDK4/6 inhibitor; and a third reference profile representing sensitivity to the CDK4/6 inhibitor but without therapeutic benefit over classical treatment by surgery and hormone therapy; - correlating the level of expression of the 11 genes or fragments thereof in the sample with the level of expression of the 11 genes or fragments thereof in the three reference profiles; - identifying the reference profile which provides the highest correlation with the level of expression of the 11 genes or fragments thereof in the sample; and - inferring from the finding that: (i) the first reference profile provides the highest correlation with the level of expression of the 11 genes or fragments thereof in the sample, that the subject is insensitive to the CDK4/6 inhibitor; (ii) the second reference profile provides the highest correlation with the level of expression of the 11 genes or fragments thereof in the sample, that the subject is therapeutically sensitive to the CDK4/6 inhibitor; or (iii) the third reference profile provides the highest correlation with the level of expression of the 11 genes or fragments thereof in the sample, that the subject is sensitive to the CDK4/6 inhibitor but has no clinical benefit from treatment with the CDK4/6 inhibitor. - determining in a sample from the subject the level of expression of 11 genes or fragments thereof consisting of cyclin-dependent kinase inhibitor 2A (CDKN2A); cyclin E1 (CCNE1); transgelin 2 (TAGLN2); translocase of inner mitochondrial membrane 17 homolog A (yeast) (TIMM17A); RAB31, member RAS oncogene family; spindle apparatus coiled-coil protein 1 (SPDL1); nucleoporin 155kDa; F-box and leucine-rich repeat protein 5 (FBXL5); gelsolin; TP53 target 1 (non-protein coding) (TP53TG1); and protein phosphatase 1 regulatory subunit 3C (PPP1R3C); - providing three reference profiles of the level of expression of the 11 genes or fragments thereof: a first reference profile representing insensitivity to the CDK4/6 inhibitor; a second reference profile representing therapeutic sensitivity to the CDK4/6 inhibitor; and a third reference profile representing sensitivity to the CDK4/6 inhibitor but without therapeutic benefit over classical treatment by surgery and hormone therapy; - correlating the level of expression of the 11 genes or fragments thereof in the sample with the level of expression of the 11 genes or fragments thereof in the three reference profiles; - identifying the reference profile which provides the highest correlation with the level of expression of the 11 genes or fragments thereof in the sample; and - inferring from the finding that: (i) the first reference profile provides the highest correlation with the level of expression of the 11 genes or fragments thereof in the sample, that the subject is insensitive to the CDK4/6 inhibitor; (ii) the second reference profile provides the highest correlation with the level of expression of the 11 genes or fragments thereof in the sample, that the subject is therapeutically sensitive to the CDK4/6 inhibitor; or (iii) the third reference profile provides the highest correlation with the level of expression of the 11 genes or fragments thereof in the sample, that the subject is sensitive to the CDK4/6 inhibitor but has no clinical benefit from treatment with the CDK4/6 inhibitor. (i) the first reference profile provides the highest correlation with the level of expression of the 11 genes or fragments thereof in the sample, that the subject is insensitive to the CDK4/6 inhibitor; (ii) the second reference profile provides the highest correlation with the level of expression of the 11 genes or fragments thereof in the sample, that the subject is therapeutically sensitive to the CDK4/6 inhibitor; or (iii) the third reference profile provides the highest correlation with the level of expression of the 11 genes or fragments thereof in the sample, that the subject is sensitive to the CDK4/6 inhibitor but has no clinical benefit from treatment with the CDK4/6 inhibitor.