ESR1 and breast cancer: Approximately 70% of breast cancers are estrogen receptor positive (ER +) and are treated with endocrine therapy (such as nonsteroidal aromatase inhibitors [NSAIs, anastrozole or letrozole] or steroidal aromatase inhibitors [exemestane, EXE]) in combination with CDK4/6 inhibitors in the first-line setting [4–6].