ESR1 and mucinous adenocarcinoma: Mucinous carcinomas can be misdiagnosed as benign lesions because of their slow growth, clear boundary, and smooth border.[3,10] Immunohistochemistry of these tumors shows that they are usually positive for estrogen receptor (94%) and progesterone receptor (80%), as well as negative for HER2 (93%).[11–13] This indicates a favorable prognosis for the disease.[13,14]