Unfortunately, despite the success rate of cell cycle checkpoint CDK4/6 inhibitors, 10% of breast cancer patients have intrinsic resistance, and many later present with acquired resistance since the median response rate among patients receiving first-line treatment of CDK4/6 inhibitors plus hormone therapy was determined to be only 24–28 months in the PALOMA, MONALEESA, and MONARCH clinical trials [7]. This evidence concerns the gene CDK4 and breast cancer.