ESR1 and breast carcinoma: Previous molecular studies demonstrated that breast cancer can be classified into Luminal A (ER+/PR+/HER2-/low Ki-67); Luminal B (ER+/PR+/HER2-/+/high Ki-67), HER2-overexpression (ER-/PR-/HER2+) and triple negative breast cancers (ER-/PR-/HER2-) (Al-Thoubaity, 2020; Goldhirsch et al., 2011).