This observation, together with the information extracted from the microarray data (Figure 7-A), highlights TACSTD2 as an important biomarker for both ER+ and ER- breast cancer subtypes, as well as an attractive candidate for drug therapy against the triple negative (ER-, PR- (progesterone receptor) and HER2-) subtype of breast cancer, with potential implications for treating drug-resistant cases that are non-responsive to ER/HER2-targeted therapies. This evidence concerns the gene ESR1 and breast cancer.