Immunohistochemical localization of mesothelin in normal hen ovaries and hen ovaries containing tumor. Staining was not observed in normal ovaries (A-C) in either the stromal or follicular (F) compartments or in ovarian tumors that did not express mesothelin (D-F). Mesothelin staining was observed in the ovarian surface epithelium (SE) and clusters of cells in the ovarian stroma (ST) of tumors that express mesothelin (G-I). Examples of different ovarian tumor histology are shown (D and F, advanced mucinous tumor; E, advanced endometrioid tumor; G and H, advanced serous and endometrioid mixed tumor respectively; I, advanced serous tumor). The original magnification was 40×.