Genetic alterations in tumor suppressor genes are believed to play an important role in the initiation of childhood and adult malignancies. Tumor-specific loss of heterozygosity for particular chromosomal regions has provided the starting point for the cloning of different tumor suppressor genes, including the Wilms tumor predisposing gene, WT1, at chromosome 11p13. This article reviews the pathology and genetics of Wilms tumor, the cloning of WT1, and the WT1 mutations reported thus far in 15 hereditary and nonhereditary Wilms tumors. The presence of constitutional WT1 mutations in 35 patients with the Denys-Drash syndrome (a syndrome consisting of nephropathy, intersex disorders, and Wilms tumor) is also described. To date, mutations in the WT1 gene have been found in less than 10% of Wilms tumors specimens examined and in greater than 95% of Denys-Drash patients. The possible significance of this observation with regard to both the cellular function of the WT1 protein and the involvement of alternative loci in the development of Wilms tumor is discussed.