Determining hormonal receptor status and the Ki-67 proliferation index are essential steps in characterizing breast cancer, including DCIS, as they form the basis for classifying tumors into four molecular subtypes: Luminal A (HR+ and Ki-67 < 14%), Luminal B (HR+ and Ki-67 ≥ 14%), HER2+ (HR− and HER2+), and basal-like (triple-negative) [23,29,30]. The gene discussed is ERBB2; the disease is ductal breast carcinoma in situ.