ESR1 and breast cancer: Multivariate Cox regression analysis showed that ER negativity (P < 0.001), a larger tumor size (P < 0.001), lymph node metastasis (P < 0.001), and young age at diagnosis (P = 0.006) were associated with unfavorable RFS in patients with BC, while the co‐occurrence of TC was not (P = 0.437).