Van Elssen C H M J, Frings P W H, Bot F J, Van de Vijver K K, Huls M B, Meek B, Hupperets P, Germeraad W T V & Bos G M J (2010) Histopathology57, 597–606 Expression of aberrantly glycosylated Mucin-1 in ovarian cancer Aims: Mucin 1 (MUC1) is an important tumour-associated antigen (TAA), both overexpressed and aberrantly glycosylated in adenocarcinomas. The aim of this study was to examine the MUC1-glycosylation status of primary ovarian adenocarcinomas and metastatic lesions. Methods and results: Paraffin-embedded tissue sections of 37 primary ovarian adenocarcinomas representing all histotypes (22 serous, five mucinous, two clear-cell, eight endometrioid), four serous borderline tumours with intraepithelial carcinoma, seven sections of ovarian endometriosis and 13 metastatic lesions were analysed by immunohistochemistry. Non-neoplastic ovarian surface epithelium and serous cystadenomas were used as controls. All epithelia expressed MUC1 protein. Of primary tumours, 76% expressed the differentiation-dependent glycoform and 84% the cancer-associated glycoform (Tn/Sialyl-Tn-epitopes). In metastatic lesions this was 77% and 85%, respectively. Notably, in 57% of ovarian endometriosis and 75% of intraepithelial lesions, the cancer-associated MUC1 epitopes were expressed, whereas normal ovarian surface epithelium and serous cystadenomas did not express these epitopes. Conclusions: The underglycosylated MUC1 epitopes are expressed by all histotypes of primary ovarian adenocarcinomas, by the vast majority of metastatic lesions and by possible ovarian cancer precursor lesions, but not by normal ovarian tissue. These results indicate that MUC1-associated Tn/STn-epitopes are important targets for immunotherapy and diagnostic imaging in ovarian cancer patients.