ESR1 and breast cancer: The advent of cyclin-dependent kinases 4/6 (CDK4/6) inhibitors (CDK4/6i) has transformed the treatment landscape in metastatic estrogen receptor–positive (ER+) breast cancer, and CDK4/6i in combination with endocrine treatment is now widely used as first- and second-line treatment in metastatic ER+ breast cancer (1).