CDK4 and breast cancer: Extensive characterization of baseline tumor tissues in PALOMA-2 using immunohistochemistry, fluorescence in situ hybridization, and gene expression panels have highlighted the importance of CDK4/6 signaling in hormone receptor–positive (HR+ )/HER2– breast cancer, revealing that the interplay between steroid hormone and peptide growth factor signaling could drive dependence on CDK4/6 signaling18.