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Background: WT1, first recognized as a tumor suppressor gene involved in the development of Wilms' tumor, may have apparently contradictory findings and functions. As WT1 has been identified as a molecular target for cancer immunotherapy, immunodetection of WT1 in tumor cells has become an essential step in cancer studies., Methods: We compare the expression of this protein among different types of melanocytic nevi and among stages in primary melanoma progression. Tissue microarrays containing normal tissues and 271 primary melanocytic lesion samples (163 primary melanomas and 108 nevi) were studied by immunohistochemistry using monoclonal antibody against WT1., Results and Discussion: The present study shows these: 1. WT1 protein is predominantly expressed in the cytoplasm of the neoplastic cells. 2. A higher rate of WT1 staining in melanocytic nevi against melanomas has been observed. 3. WT1 expression is increased in advanced stages of melanoma progression: a significant (p < 0.05) increase of expression of WT1 was detected in vertical cases 46.5% vs. radial cases 16.0%, in high levels of Clark (IV, V) 57.4% vs. low levels (I, II, III) 33.0% and when comparing depth of invasion within thickness subgroups. 4. Finally, this study establishes an association of WT1 protein expression with shorter overall survival in melanoma.