A genetic description of the human genome requires maps of three types. The first shows the frequency of chromosomal interchange during meiosis, relying on many equally spaced markers, and is limited to interchanges that do not unmask defects lethal to the conceptus, whose every cell will contain such abnormalities. The second is the physical description of genomic regions defined by karyotypic rearrangements, DNA segments, genes, or their products. A third description of somatic chromosomal interchanges at mitosis is also required. Because mitotic exchanges occur in a single postembryonic somatic progenitor cell, lethal effects on the organism are reduced. These events have been important in genetic mapping in Drosophila melanogaster and fungi, but they have rarely been detected in mammals. Here we report a significant frequency of mitotic recombination in human tumours and the first application of this information in localizing their predisposing locus.