Clinical evidence suggests that despite favorable prognostic indicators, e.g., ER+ and/or progesterone receptor positive, low Ki67 proliferation index and HER2− status, patients with ILC have similar or worse long-term outcomes compared to those with invasive ductal carcinoma (IDC, otherwise known as invasive carcinoma of no special type) [5, 6]. This evidence concerns the gene ESR1 and invasive ductal breast carcinoma.