ESR1 and breast cancer: The molecular classification of breast cancer can be divided into four types: luminal A (HR+ [ER+ and/or PR+], HER2-), luminal B (HR+ [ER+ and/or PR+], HER2+/-), HER2-positive (HR- [ER-, PR-], HER2+), and triple-negative breast cancer (TNBC; HR- [ER-, PR-], HER2-).