ESR1 and breast cancer: Importantly, the global evaluation of endocrine-resistant breast cancers suggests a taxonomy categorizing these tumors into four groups: (i) tumors bearing ESR1 mutations; (ii) tumors with molecular alterations at the level of the MAPK pathway; (iii) tumors with genetic alterations at the level of the molecular machinery involved in transcriptional regulation; and, (iv) pan-wild type tumors with an unknown mechanism of resistance [167].