Wilms' Tumor 1 (WT1) is a transcription factor that plays an important role in cellular development and cell survival. The WT1 gene encodes a tumor suppressor gene inactivated in Wilms' tumor, recently implicated in WNT signaling through the enhancement of cytoplasmic beta-catenin (CTNNB1) degradation. WT1 has been demonstrated in mesenchymal-derived cells and in Wilms' tumor. An antibody to WT1 is useful for the identification of malignant mesothelioma. A literature review of 88 published papers suggested that the sensitivity and specificity of WT1 for the identification of epithelioid mesothelioma was 77% and 96%, respectively. WT1 immunoreactivity has also been detected in several types of other malignancies, including peritoneal carcinoma, breast carcinoma, ovarian carcinoma and leukemia. In hepatocellular carcinoma, the expression of WT1 is correlated with a response to chemotherapy. Additionally, WT1 is a useful marker to differentiate desmoplastic small round cell tumors (DSRCT) from other small round cell tumors.