Alongside recent approval and growing clinical use of CDK4/6 inhibitors like Palbociclib in combination with endocrine agents to treat metastatic ER-positive breast cancers, there have been parallel advancements in the clinical development of CDK2-selective inhibitors like Dinaciclib (Merck) and CYC065 (Cyclacel) for the treatment of hematopoietic and MYC-activated malignancies and potentially also breast cancers [19–22]. This evidence concerns the gene CDK4 and breast cancer.