The human genetics revolution demands an ever-increasing quest for further information about genetic testing, its uses and availability for specific disorders from healthcare providers, the media and other users of genetic services.

Genetic Testing involves examining a person's DNA - taken from cells in a sample of blood or, occasionally, from other body fluids or tissues - for some anomaly that flags a disease or disorder. The DNA change can be relatively large: a missing or added piece of a chromosome - even an entire chromosome - that is visible under a microscope. Or it can be extremely small, as little as one extra, missing, or altered chemical base. Genes can be over-expressed (too many copies), inactivated, or lost altogether. Sometimes, pieces of chromosomes become switched, or transposed, so that a gene ends up in a location where it is permanently and inappropriately turned on or off.

In addition to studying chromosomes or genes, genetic testing in a broader sense includes biochemical tests for the presence or absence of key proteins that signal aberrant genes.

Inherited disorder testing can be used to look for possible predisposition to disease as well as to confirm a suspected mutation in an individual or family.

The most widespread type of genetic testing is newborn screening whereby newborn infants have blood samples tested for abnormal or missing gene products. Some tests look for abnormal arrangements of the chemical bases in the gene itself, while other tests detect inborn errors of metabolism (for example, phenylketonuria) by verifying the absence of a protein that the cell needs to function normally.

Carrier testing can be used to help couples to learn if they carry - and thus risk passing to their children - a recessive allele for inherited disorders such as cystic fibrosis, sickle-cell anaemia, or Tay-Sachs disease (a lethal disorder of lipid metabolism).

In clinical research programs, doctors make use of genetic tests to identify telltale DNA changes in cancer or precancer cells. Such tests can be helpful in several areas: early detection (familial adenomatous polyposis genes prompt close surveillance for colon cancer); diagnosis (different types of leukaemia can be distinguished); prognosis (the product of a mutated p53 tumour-suppressor gene flags cancers that are likely to grow aggressively); and treatment (antibodies block a gene product that promotes the growth of breast cancer).

Much of the current excitement in gene testing, however, centres on predictive gene testing: tests that identify people who are at risk of getting a disease, before any symptoms appear. Tests are already available in research programs for some two dozen such diseases, and as more disease genes are discovered, more gene tests can be expected.

The benefits of gene testing for persons in high-risk families living with troubling uncertainties about their own future as well as that of their children is that a negative test - especially one that is strongly predictive - can create a tremendous sense of relief.

A negative test, especially one that is strongly predictive, also may eliminate the need for frequent checkups and tests such as annual colonoscopy (a procedure that allows a physician to view the upper reaches of the large intestine), which are routine for high-risk families concerned about cancer.

A positive test can also produce benefits. It can relieve uncertainty, and it can allow a person to make informed decisions about his or her future.

Under the best of circumstances, a positive test creates an excellent opportunity for counselling and interventions to reduce risk. The prime example is colon cancer. When tumours are caught early, chances for survival are greatest, and screening potentially could prevent thousands of cancer deaths a year. A positive gene test sounds the alert to keep up regular screening practices (annual colonoscopies to check for precancerous polyps or the earliest signs of cancer) and to maintain healthful lifestyle measures such as a high-fibre, low-fat diet and regular exercise. Another option is surgery to remove the colon before cancer has a chance to develop.

Further information can be found at http://www.accessexcellence.org/AE/AEPC/NIH/index.html