Mitotic events leading to the loss of the normal allele corresponding to a mutated gene are important for tumorigenesis in rare heritable tumors such as retinoblastoma and Wilms tumor. As reported for both colorectal and breast cancers, some common tumors seem to develop because of the same mitotic events. We examined constitutional and tumor genotypes defined by polymorphic DNA clones in 36 patients with gastric cancer. In 14 cases, constitutional heterozygosity at loci on chromosome 13 had been lost. Loss of alleles was also detected at a locus on chromosome 18 in two cases and at a locus on chromosome 17 in one case. The frequent loss of alleles at loci on chromosome 13 (41%) suggests that elimination of genes on this chromosome may be of importance in the tumorigenesis of human primary gastric cancers.
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