ESR1 and mismatch repair cancer syndrome 1: MMR deficiency is significantly associated with inferior overall (HR 2.29, 95% CI 1.02–5.17, p = 0.040) and disease-specific survival (HR 2.71, 95% CI 1.00–7.35, p = 0.042) in the 431 estrogen receptor-positive patients who were uniformly treated with tamoxifen as their sole adjuvant systemic therapy.