ESR1 and neoplasm: The combination of a cyclin-dependent kinase 4 and 6 (CDK4/6) inhibitor with endocrine therapy (ET) is the current standard of care for patients with hormone receptor-positive (HR+) human epidermal growth factor receptor 2-negative (HER2−) metastatic breast cancer (MBC) which is defined by the expression of either the estrogen receptor (ER) or progesterone receptor (PgR) in at least 1% of tumor cells and the absence of HER2 overexpression or amplification [1,2].