In both cohorts, most of the primary tumors were invasive breast carcinoma of no special type (IBC-NST) (86%) and expressed ER but not HER2 (Table 1, Supplementary Table 3), with 72% (76/106) primary cancers being ER+/HER2non-amp, 5% (5/106) ER−/HER2amp, 12% (13/106) ER+/HER2amp and 11% (12/106) ER−/HER2non-amp. Here, ESR1 is linked to invasive breast carcinoma.