ESR1 and breast cancer: Clinical BC types include hormone receptor (HR)-positive (HR+/HER2-, also ER+/PR+/HER2-), triple positive (HR+/HER2+, also ER+/PR+/HER2+), HER2-positive (ER-/PR-/HER2+, also HER2+), and triple-negative breast cancer (TNBC) (HR-/HER2-, also ER-/PR-/HER2-) [2].