Ever since scientists began to decipher the human genome in the 1990s, hopes have run high that unraveling the genetic code would rapidly revolutionize health care. But while gene research has changed scientists' understanding of many diseases, practical applications remain few. Genetic tests for single-gene diseases such as cystic fibrosis are available, and doctors can more precisely pinpoint the kind of cancer a patient has by analyzing genes. Gene tests can also predict how patients will respond to some drugs. However, no actual gene therapy has been approved for human use, and researchers now know that diseases such as heart disease, Alzheimer's and adult-onset diabetes usually result from a complex interaction among numerous genes and a person's environment. These findings cast doubt on whether current genetic tests for such prevalent diseases are accurate and whether gene science makes it any easier to find treatments.