Recent molecular studies have shown a relatively high rate of loss of heterozygosity (LOH) at band 7p15-21 in Wilms tumor. We previously reported that the minimal common region of LOH was located between markers D7S517 and D7S503 in bands 7p15-21. We also reported the identification of one Wilms tumor (GOS44) bearing a homozygous, interstitial deletion at a locus within this region. Homogeneous primary cell cultures have been derived from this tumor and have been used for all the subsequent analyses. Using PCR and a panel of STS markers mapping between D7S517 and D7S503, the physical boundaries of the homozygous deletion were determined to be between D7S638 and D7S644. The deleted region spans approximately 3 Mbp of genomic sequence and includes seven known genes (KIAA0744, KIAA0713, AHR, AGR2, NET6, HSPC028, and DGKB.) as well as five predicted genes with similarities to genes of known function (LOC-91802, -116364, -96009, -92511, and -92512). The proximal breakpoint was found to lie between exon 6 and exon 7 of KIAA0744, and the distal breakpoint lay between exon 17 and exon 18 of DGKB. It is unlikely that a functional fusion gene product was generated as a consequence of the fusion between these two genes, because they are oriented in opposite directions on the chromosome. This is the only reported homozygous deletion recorded so far in Wilms tumor, and it provides the means to identify the tumor-suppressor gene located in this deletion.