<b>Background:</b> Body mass index (BMI) has been widely investigated as a potential prognostic factor in breast cancer; however, its clinical relevance in patients with hormone receptor-positive/human epidermal growth factor receptor 2-negative (HR+/HER2-) metastatic breast cancer treated with CDK4/6 inhibitors remains controversial, particularly in contemporary real-world settings. Here, ERBB2 is linked to breast cancer.