The International HapMap Project was de-signedtocreateapublic, genome-wide database of patterns of common human sequence variation to guide genetic studies of human health and disease, including genome-wide association (GWA) studies (1, 2). Identifying genetic influences on complex diseases would be quite difficult if the risk-associated allelic variants at a particular disease-causing locus were very rare, so that for a disease to be common there would be many different causative alleles. The HapMap was instead designed to facilitate identification of commonly occurring disease-causing variants based upon the “common disease, common variant” hypothesis (3). This hypothesis suggests that at least some of the genetic influences on many common diseases are attributable to a limited number of common allelic variants that are present in more than 5% of the population.
