ESR1 and breast cancer: AI: aromatase inhibitor; CT: chemotherapy; eBC: early breast cancer; ER: estrogen receptor; ET: endocrine therapy; FSH: follicle-stimulating hormone; HER2-: human epidermal growth factor receptor 2-negative; HR+: hormone receptor-positive; Ki-67: antigen Kiel 67; LHRHa: luteinizing hormone releasing hormone analog; OFS: ovarian function suppression; RANK: receptor activator of nuclear factor-kappaB