Invasive ductal carcinoma is the most common breast malignancy and a leading cause of cancer-related death in women worldwide.[1] Despite developments in surgical methods, cytotoxic chemotherapy, and agents targeted against estrogen receptor (ER) and HER2, a subset of patients with advanced stage invasive ductal carcinoma may experience tumor recurrence or metastasis within several years after treatment. The gene discussed is ESR1; the disease is invasive ductal breast carcinoma.