For postmenopausal women with estrogen receptor-positive/human epidermal growth factor receptor 2-negative (ER+/HER2−) advanced breast cancer (ABC), a cyclin-dependent kinase (CDK4/6) inhibitor combined with an aromatase inhibitor (AI) is strongly recommended as first-line therapy by the Japanese Breast Cancer Society [6]. This evidence concerns the gene CDK4 and breast cancer.