The Human Genome Project, completed in 2003, revealed just how much individual variation there is. Researchers worked out the order (or sequence) of the three billion DNA bases (chemical building blocks) that constitute the human genome (the complete set of human DNA). Although it's about 99% the same in all people, it still varies at more than 10 million DNA bases. That variation explains, in part, our varying degrees of risk for certain diseases.
In medical settings, genetic tests have been used to identify variations that cause serious health conditions. These tests are usually reserved for people known to be at risk for a specific disease because it runs in families. For example, couples planning a pregnancy may be tested to determine whether they carry the gene for Tay-Sachs disease. Women with close relatives who developed breast cancer early in life may want to know if they carry one of the high-risk BRCA genes. Because the results of such tests can alter lives, they are best administered only after individuals have been counseled on the risks, benefits, and limits of testing and have given informed consent. The results are confidential, and their implications should be explained to patients by genetic counselors.
Clinicians can also use genetic testing to help them select more effective drug treatments. For example, postmenopausal women with breast cancer for whom tamoxifen may be an option are sometimes tested to see if they have a gene variant that renders tamoxifen less effective; if they do, they can be prescribed a drug that works differently. Another genetic test may help determine whether patients at risk for blood clotting will benefit more from clopidogrel (Plavix) or from another drug such as prasugrel (Effient).