ESR1 and medical procedure: Regarding immunohistochemical features of breast NENs, most cases are positive for somatostatin receptor 2 as well as the estrogen receptor, and E-cadherin, while being negative for HER1 (EGFR), HER2, occasionally with gene analysis, and high-molecular-weight cytokeratins (CKs), such as CK5/6 and CK14 (9, 22–24).