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Byline: Paul Thorner (1), Jeremy Squire (2), Natasha Plavsic (1), Roland Jong (1), Mark Greenberg (3), Maria Zielenska (1) Keywords: Key words:WT1, polyphenotypic tumor, mesothelioma Abstract: The WT1 gene is normally expressed in fetal kidney and mesothelium, and its expression has been suggested as a marker for Wilms tumor and mesothelioma. We examined WT1 expression levels by reverse-transcriptase polymerase chain reaction (RT-PCR) in 38 childhood small-cell tumors including Wilms tumor, embryonal and alveolar rhabdomyosarcoma, Ewing sarcoma, lymphoma, desmoplastic small round-cell tumor (DSRCT), synovial sarcoma, extrarenal rhabdoid tumor, and two tumors that were atypical for this group of tumors. WT1 expression was only detected in Wilms tumor, rhabdoid tumor, and in these two cases of uncertain histogenesis. Both arose in the peritoneal cavity and by immunohistochemistry were diffusely positive for vimentin, keratin, and desmin. Tonofilaments were identified by electron microscopy in one of the cases. RT-PCR failed to detect the t(11 22) translocation associated with DSRCT in either case. Our results suggest that WT1 expression is an unusual feature of childhood non-Wilms tumors and, in the right setting, it may indicate a mesothelial origin. The expression of WT1 may play a role in mesodermal cells acquiring epithelial characteristics, a concept supported by the mixed epithelial and mesenchymal phenotype of these two cases. Author Affiliation: (1) Department of Pediatric Laboratory Medicine, Division of Pathology, The Hospital for Sick Children, 555 University Avenue, Toronto, Ontario, Canada M5G 1X8 , CA (2) Division of Cellular and Molecular Biology, The Ontario Cancer Institute, 610 University Avenue, Toronto, Ontario, Canada M5G 2M9 , CA (3) Department of Pediatrics, Division of Oncology, The Hospital for Sick Children, 555 University Avenue, Toronto, Ontario, Canada M5G 1X8 , CA Article note: Received November 28, 1997 accepted May 7, 1998.