ESR1 and invasive lobular breast carcinoma: Immunohistochemical staining demonstrated positivity for AE1/AE3 (pan-cytokeratin antibody cocktail) and CAM5.2 (low molecular weight cytokeratin antibody), and loss of E-cadherin expression, findings consistent with invasive lobular carcinoma of the breast. Hormone receptor analysis showed that the estrogen receptor (ER) was expressed in more than 95% of tumor cells, the progesterone receptor (PgR) was not expressed (0%), the HER2 score was 1+, and the Ki-67 labeling index was 35%.