We describe the partial loss of heterozygosity (LOH) at chromosome 11 p loci in normal tissues (normal kidney and/or blood) from four of 67 Wilms' tumour patients. Autologous tumour DNA showed complete loss of the same, maternally derived, alleles. These observations indicate that the normal tissues were mosaic for cells heterozygous and homozygous for 11 p markers and that tumours subsequently developed from the homozygous cells that had undergone an 11 p somatic recombination event. We suggest that LOH for 11 p alleles is compatible with normal growth and differentiation and is significant pathologically only when accompanied by other genetic alterations.
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