In this preclinical study, by using a panel of breast cancer cell lines including T47D, MCF7, MDA-MB-361, SK-BR-3, HCC1954, MDA-MB-468, MDA-MB-231, and BT-549 (representing ER/PR+/−, HER2+, or triple negative, respectively) (Table 1)26, 27, 28, we examined the cytotoxic effects of MLN9708 and whether MLN9708 could sensitize breast cancer cells to Dox-induced apoptosis. The gene discussed is ESR1; the disease is breast cancer.