ESR1 and breast cancer: Recently, using an RNA in situ hybridization (ISH) assay that targets both epithelial and mesenchymal markers, Yu et al. surprisingly showed that the EMT characteristics being reported in TNBC can be attributed to a higher percentage of E/M hybrid breast cancer cells in TNBC compared to the other subtypes, ER/PgR+ and HER2+ , in which epithelial phenotype cells were dominant21.