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Loss of imprinting at insulin-like growth factor II (IGFII), in association with H19 silencing, has been described previously in a subgroup of Beckwith-Wiedemann syndrome (BWS) patients who have an elevated risk for Wilms' tumor. An equivalent somatic mutation occurs in sporadic Wilms' tumor. We describe a family with overgrowth in three generations and Wilms' tumor in two generations, with paternal inheritance of a cis-duplication at 11p15.5 spanning the BWS IC1 region and including H19, IGFII, INS, and TH. The duplicated region was below the limit of detection by high-resolution karyotyping and fluorescence in situ hybridization, has a predicted minimum size of 400 kb, and was confirmed by genotyping and gene-dosage analysis on a CytoChip comparative genomic hybridization bacterial artificial chromosome array. IGFII is the only known paternally expressed oncogene mapping within the duplicated region and our findings directly implicate IGFII in Wilms' tumorigenesis and add to the mutation spectrum that increases the effective dose of IGFII. Furthermore, this study raises the possibility that sporadic cases of overgrowth and Wilms' tumor, presenting with apparent gain of methylation at IC1, may be explained by submicroscopic paternal duplications. This finding has important implications for determining the transmission risk in these disorders.