A complete physical map of the 11p13 region containing the Wilms' tumor locus has been developed and used to localize a candidate Wilms' tumor gene. Our strategy to construct the map combined the use of pulsed-field gel electrophoresis and irradiation-reduced somatic cell hybrids. These hybrids, which contain limited segments of human chromosome 11 segregated from the remainder of the human genome, permit direct visualization of restriction fragments located in 11p13 using human interspersed repeated DNA sequences as hybridization probes. The physical map has provided a framework to identify the sites of genes responsible for the complex of disorders associated with hemizygous 11p13 deletion: Wilms' tumor, aniridia, genitourinary abnormalities, and mental retardation. The Wilms' tumor locus has been limited to a region of less than 345 kb, and a transcript with many of the characteristics expected for the Wilms' tumor gene has been localized to this region.