In gene tests, scientists scan a patient's DNA sample for mutated sequences. A DNA sample can be obtained from any tissue, including blood. For some types of gene tests, researchers design short pieces of DNA, called probes, whose sequences are complementary to the mutated sequences. These probes will seek their complement among the three billion base pairs of an individual's genome. If the mutated sequence is present in the patient's genome, the probe will bind to it and flag the mutation. Another type of DNA testing involves comparing the sequence of DNA bases in a patient's gene to a normal version of the gene. Cost of testing can range from hundreds to thousands of dollars, depending on the sizes of the genes and the numbers of mutations tested.

Gene testing already has dramatically improved lives. Some tests are used to clarify a diagnosis and direct a physician toward appropriate treatments, while others allow families to avoid having children with devastating diseases or identify people at high risk for conditions that may be preventable. Aggressive monitoring for and removal of colon growths in those inheriting a gene for familial adenomatous polyposis, for example, has saved many lives. On the horizon is a gene test that will provide doctors with a simple diagnostic test for a common iron storage disease, transforming it from a usually fatal condition to a treatable one.

The recently commercialized gene tests for adult-onset disorders such as Alzheimer's disease and some cancers are the subject of most of the debate over gene testing. These tests are targeted to healthy (presymptomatic) people who are identified as being at high risk because of a strong family medical history for the disorder. The tests give only a probability for developing the disorder. One of the most serious limitations of these susceptibility tests is the difficulty of interpreting a positive result because some people who carry a disease-associated mutation never develop the disease. Scientists believe that these mutations may work together with other, unknown mutations or with environmental factors to cause disease.

A limitation of all medical testing is the possibility for laboratory errors. These might be due to sample misidentification, errors resulting from contamination of the chemicals used for testing, or other sources.

Many in the medical establishment feel that uncertainties surrounding test interpretation, current lack of available medical options for these diseases, their potential for provoking anxiety, and risks for discrimination and social stigmatization could outweigh the benefits of testing. Some complexities of current gene tests are outlined below in discussions of tests for Huntington's disease and cystic fibrosis, two disorders caused by single-gene defects, and the tests that may detect predispositions to the more complex Alzheimer's disease and breast cancer. [8/98]