To investigate the antitumor effect of bortezomib on breast cancer cells, we first assessed the apoptotic effect of bortezomib in a panel of five human breast cancer cell lines: TNBC cells HCC-1937, MDA-MB-231, and MDA-MB-468; HER2-overexpressing cells MDA-MB-453; and estrogen receptor positive cells MCF-7. The gene discussed is ESR1; the disease is breast cancer.