Abbreviations: DCIS = ductal carcinoma in situ; IDC = invasive ductal carcinoma; ILC = invasive lobular carcinoma; IDLC = invasive ductal-lobular carcinoma; ER = estrogen receptor; PR = progesterone receptor; HER2 = human epidermal growth factor receptor 2; TN = triple negative. The gene discussed is ESR1; the disease is invasive lobular breast carcinoma.