ESR1 and breast carcinoma: Triple-negative breast cancer (TNBC), defined as lacking expression of ER/PR/HER2 receptors, represents 15–20% of breast cancer, and it is associated with the highest probability of relapse among breast cancer subtypes despite local treatments and cytotoxic chemotherapy.4 The majority of TNBCs are classified as BL and vice versa, with an overlap between the two classifications of ~80%.5 The broad heterogeneity of TNBC, both inter- and intra-tumoral, has contributed to the difficulties in successfully treating it.