ESR1 and neoplasm: In the COX multivariate analysis including the standard prognostic factors i.e., tumor size (T2-4 vs. T1), nodal status (N1-3 vs. N0), grade, ER/PR-status, and adjuvant trastuzumab therapy, the significant prognostic factors for OS were adjuvant trastuzumab (HR 0.29, 95% CI 0.15–0.55, p < 0.001) and tumor size (HR 2.06, 95% CI 1.12–3.78, p = 0.020).