In patients with hormone-receptor-positive and human epidermal growth factor receptor 2 (HER-2)-negative metastatic breast cancer without imminent organ failure, current standard first-line therapy includes a combination of endocrine therapy, such as aromatase inhibitors or tamoxifen and cyclin-dependent kinase (CDK) 4/6 inhibitors [3]. This evidence concerns the gene ERBB2 and breast cancer.