ESR1 and mismatch repair cancer syndrome 1: In survival analyses stratified by treatment (Fig. 3), MMR deficiency was associated with significantly shorter survival within the cohort of ER-positive patients treated with tamoxifen as their sole adjuvant therapy (OS: HR 2.29, p = 0.040, n = 431; DSS: HR 2.71, p = 0.042, n = 431).