Genetic factors contribute to virtually every human disease, conferring susceptibility or resistance, or influencing interaction with environmental factors. Much research in both the public and private sectors is driven by the expectation that understanding the genetic contribution to disease will revolutionize diagnosis, treatment, and prevention. Understanding the role played by genetic factors in disease is also expected to increase understanding of the nongenetic, environmental contributions.

The genetic maps, physical maps, and technologies for gene identification that have emerged from the Human Genome Project (HGP) have already had a significant effect on the research community's ability to discover genes underlying Mendelian disorders. Positional cloning (1), in which a disease gene is identified by virtue of its location in the genome rather than by using knowledge of its biochemical function, was successfully applied in the search for human genes for the first time in 1986. By 1990, when the HGP began, only a handful of additional successes had accrued. By 1997, however, close to 100 disease loci have been identified with this strategy.