ESR1 and breast carcinoma: Cyclin-dependent 4/6 kinase inhibitors (CDK4/6i) in combination with endocrine therapy have emerged as the standard first-line treatment for advanced (metastatic or unresectable) breast cancers that are estrogen and/or progesterone receptor-positive (ER+) and Human Epidermal Growth factor Receptor-2-negative (HER2-) [1,2,3].