CDK4 and breast cancer: It has already been shown that alterations in cell cycle genes and proteins are associated with acquired resistance to palbociclib.8–10,32,33 In particular, cyclin E overexpression and Rb loss have been previously indicated as possible mechanisms of acquired resistance to CDK4/6 inhibitors in some BC cell lines.8–10 Our GE and WB data showed that both cyclin E1 overexpression and Rb underexpression occur across different and heterogeneous BC cellular models, including HER2+ cells, which has not been shown previously.