Breast cancer cells can be characterized based on the presence and absence of various receptors like estrogen receptor (ER), progesterone receptor (PR), epidermal growth receptor (HER2), etc. This study is based on the highly metastatic triple-negative breast cancer cell lines, MDA-MB-231 (ER-, PR-, and HER2-), isolated from basal-type carcinoma of the mammary gland [3]. The gene discussed is ESR1; the disease is breast cancer.