CDK4 and breast cancer: HR-positive breast cancer represents the most frequent breast cancer subtype, and cyclin dependent kinase 4 and 6 (CDK4/6) inhibitor combined with aromatase inhibitors or tamoxifen is the current standard frontline therapy to treat such cancer types and CDK4/6 inhibitors combined with fulvestrant is a preferred second line therapy for the endocrine-resistant breast cancer patients [3].