ESR1 and breast cancer: Statuses of these receptors define four main subgroups of breast cancer (BC): Luminal A (ER- and PR-positive and HER2-negative), Luminal B (ER-, PR-, and HER2-positive), HER2 overexpressed (ER- and PR-negative and HER2-positive), and triple-negative breast cancer (TNBC) (ER-, PR-, and HER2-negative) [109].