CDK4 and metastatic malignant neoplasm: We can speculate that germline-mutated and Cluster 1 MBC cases, characterized by the activation of the cell cycle pathway, might benefit from the use of CDK4/6 inhibitors in combination with endocrine therapy, which is a treatment approved for MBC patients with ER/PR-positive, HER2-negative advanced or metastatic cancer [33,35].