Mesothelin is a cell surface glycoprotein that elicits T cell responses, first described as the antigenic target of the K1 monoclonal antibody, generated by using an ovarian cell line as immunogen (1,7). In normal tissues, mesothelin expression is largely restricted to mesothelial cells. However, immunoreactivity for this marker has also been reported in some epithelial cells of the trachea, tonsil, kidney, and fallopian tube (1). Mesothelin is one of the most sensitive markers for mesothelioma, and it has been reported to be expressed in all or nearly all epithelioid mesotheliomas as well as in some carcinomas, particularly those arising in the ovary (serous carcinomas) and pancreas (1-7). Epithelioid mesotheliomas and adenomatoid tumors invariably express mesothelin, and the reaction is usually strong and diffuse and occurs along the cell membrane. This is in contrast to sarcomatoid mesotheliomas, which are usually negative for this marker (6,8). Among carcinomas, most pancreatic adenocarcinomas (86%-100%) and nonmucinous carcinomas of the ovary, including serous carcinomas (93%-100%), clear cell carcinomas (43%-75%), and transitional cell carcinomas (100%), have been reported to be mesothelin positive (8). Approximately 40% to 50% of pulmonary adenocarcinomas and 15% to 30% of squamous cell carcinomas of the lung have also been reported to express this marker in a focal and cytoplasmic staining pattern, in contrast to mesotheliomas’ membranous pattern (4,5,8). Even with the low specificity for mesothelioma, the common strong membranous reactivity in epithelioid mesotheliomas should be regarded as a strong indication against the diagnosis of mesothelioma when a negative stain is achieved (3). In addition, because mesothelin expression has been reported to be either negative or rarely weakly positive in some carcinomas, such as renal cell carcinomas, it may be included in the panel of markers used to distinguish these tumors from epithelioid mesotheliomas (5,8).