The current National Comprehensive Cancer Network (NCCN) treatment guidelines and the Japanese Breast Cancer Society Clinical practice guidelines recommend a cyclin-dependent kinase 4/6 (CDK4/6) inhibitor in combination with endocrine therapy (ET) for the treatment of patients with hormone receptor‒positive (HR +)/human epidermal growth factor receptor 2‒negative (HER2‒) metastatic breast cancer (MBC) [2, 3]. This evidence concerns the gene HR and breast cancer.