ESR1 and breast cancer: Recently, it was revealed that TROP-2 could be a potential biomarker and an attractive target for drug-MAb therapy against estrogen receptor positive (EsR+) and estrogen receptor negative (EsR−) breast cancers, including the triple-negative subtypes.30 Moreover, it was found that 99% of breast cancers have this TACSTD2 expression in its ON state, suggesting it to be a better target for breast cancer therapy.