We analyzed transcriptomic and genomic data of ER+/HER2− breast cancers with in silico RS < 26 from TCGA (n = 530), two microarray cohorts (A: n = 865; B: n = 609), the METABRIC (n = 867), and the SCAN-B (n = 1636) datasets. Here, ESR1 is linked to breast carcinoma.