A significant problem in the development and reliability of genetic tests for disease predisposition is the detection of heterozygous mutations. In most cases, a mutation is found only on one chromosome, and the presence of the wild-type allele on the homologous chromosome produces misleading results. In a recent issue of Nature (403, 723–724, 2000), researchers describe a new system for converting a diploid chromosome complement to a haploid state, allowing highly reliable testing for mutations. Bert Vogelstein and his collaborators fused lymphocytes from human blood samples to mouse cells, allowing the human chromosomes to be transferred into the mouse cells. In over a quarter of the fused cells, human chromosomes had been transferred without their homologs, resulting in an artificial haploid for that chromosome. In 22 patients with hereditary non-polyposis colorectal cancer, conventional testing on diploid cells revealed disease-causing mutations in only 10 of the patients. Using the new technique, disease-causing mutations were identified in all of the patients. Vogelstein explains that the new method is “not a replacement for conventional methods of genetic testing. It simply provides improved templates for all of those kinds of tests.” The new approach is being applied to their ongoing diagnostics research, and Vogelstein expects the technology to be marketed widely within a few months.