ESR1 and ductal breast carcinoma in situ: IHC using CK5/6, CK14, and ER is useful for discrimination of intraductal epithelial proliferative lesions [13]; however, some pitfalls should be noted: 1) a pure benign papillary lesion is often negative for HMW-CK; 2) apocrine lesions are negative for HMW-CK and ER irrespective of their benign/malignant status, and IHC should not be used for discrimination of this status for apocrine lesions; and 3) high-grade DCIS, which is easily diagnosed as malignant, is frequently HMW-CK-positive and ER-negative.