Abbreviations: BMI, body mass index; DCIS, ductal carcinoma in situ; ER, estrogen receptor; HER2-positive, human epidermal growth factor receptor-2-positive breast cancer; IDC, invasive ductal carcinoma; ILC, invasive lobular carcinoma; Ki67, percentage of Ki67 proliferation marker-positive cells within the tumor; LCIS, lobular carcinoma in situ; PR, progesterone receptor. The gene discussed is ERBB2; the disease is invasive ductal breast carcinoma.