ESR1 and carcinoma: Triple negative breast cancers (TNBCs) are a pathologically and clinically heterogenous group of carcinomas defined by a lack of expression for estrogen receptor, progesterone receptor and human epidermal growth factor receptor 2 (HER2).1,2 Their proclivity for metastases notwithstanding, the diagnosis of TNBC in the metastatic setting is inherently challenging, owing to the lack of staining for hormone receptors and many breast-specific immunohistochemical (IHC) markers.