While a direct-to-consumer genetic test can estimate your risk, it cannot tell you for certain whether you will or will not develop certain forms of cancer. Many other factors, including sex, age, diet and exercise, ethnic background, a history of previous cancer, hormonal and reproductive factors, and family history also contribute to a person’s overall cancer risk.

The U.S. Food and Drug Administration (FDA) has allowed at least one direct-to-consumer genetic testing company, 23andMe, to offer a test for cancer risk. The test identifies specific genetic changes that are associated with particular cancers. For example, it looks for 44 specific genetic variants (also known as mutations) in two genes: BRCA1 and BRCA2. The variants are associated with an increased risk of breast cancer, ovarian cancer, and potentially other forms of cancer.

Researchers estimate that 5 to 10 percent of all cancers run in families. Some of these cancers are associated with inherited variants in particular genes, such as BRCA1 or BRCA2. More than 1,000 variants in each of these genes have been associated with an increased risk of cancer. However, the FDA-approved direct-to-consumer genetic test analyzes only 44 of these genetic changes.

The test offered by 23andMe also looks for two variants in the MUTYH gene. These variants can cause a condition called MUTYH-associated polyposis (MAP; also known as autosomal recessive familial adenomatous polyposis). MAP greatly increases a person's risk of  developing colorectal cancer, but it accounts for less than 1 percent of colorectal cancer cases. The two variants included in the test are the most common MAP-associated changes in people of European descent; however there are more than 100 variants in the MUTYH gene known to be associated with an increased risk of developing cancer.

Because the variants included in these tests are uncommon, most people will have a negative result. A negative result does not mean that you will never get cancer. Similarly, a positive result (one that indicates a cancer-related genetic variation) does not mean that you will definitely develop cancer.

Direct-to-consumer genetic testing for cancer risk can be stressful and anxiety-producing. Health professional organizations and patient advocacy groups strongly recommend that people considering genetic testing for cancer-associated gene variants, including those included in direct-to-consumer genetic tests, talk with a genetic counselor about the reasons they want to undergo testing and what the results could mean for their health.