The prognostic value of ER-β in TNBC has been examined in a large number of studies, but the results are conflicting, which has been attributed to the differences of: (1) type of ER-β examined (ER-β1 to ER-β5); (2) intracellular component estimated (nuclear staining or cytoplasmic staining); (3) antibody used in immunohistochemistry; (4) population of involved patients (age, stage, type of systemic therapy); (5) cancer cell type, cancer stem cells or differentiated cancer cells; (6) TP53 status (wild or mutant). This evidence concerns the gene ESR2 and cancer.