ESR1 and breast cancer: Breast cancer may be grouped into four kinds premised on their molecular subtypes: HER2-positive breast cancer (HER2+, ER–, and PR–);basal-like (ER–, PR–, and HER2–);luminal B (ER+, PR+, HER2–, and Ki67 ≥ 14% or ER+, PR+, and HER2+); and luminal A (estrogen receptor [ER] +, progesterone receptor [PR] +, HER2–, and Ki67 (proliferation marker < 14%)).