Research organizations in England and the United States that have made data from genome-wide association studies of various diseases publicly available on their Web sites have removed such information after researchers found that it is possible to determine whether a specific individual's DNA is present in a mixture of DNA samples from a thousand or more persons (Homer N et al. PLoS Genet. 2008;4:e1000167).
Genome-wide association studies use thousands of DNA variants called single nucleotide polymorphisms (SNPs) to compare groups of individuals with and without a particular disease. Using the new technique, someone who has access to an individual's SNP profile could discern whether that person's DNA is present in a pool of DNA samples.
Stephenson J. Genetic Privacy. JAMA. 2008;300(15):1752. doi:10.1001/jama.300.15.1752-c