Why Gene Tests for Cancer Don't Offer More Answers

Genetic tests for cancer have come a long way since they first entered the clinic in 1995. Back then, mutations in two genes—known as BRCA1 and BRCA2—hinted at the crucial role that genetics can play in treatment decisions. Women carrying one of those mutations (and having a family history of breast or ovarian cancer) were much more likely than the general population to develop tumors in their breasts or ovaries. Then, as now, some of these women opted to have their breasts and ovaries removed before any malignant growths could arise.

In the intervening decades, researchers have come to recognize that most cancers are driven largely by abnormalities in genes. Genetic analysis of tumors has, therefore, become standard practice for many malignancies—such as breast, lung and colon cancer—because the information may help guide therapy. Clinicians have amassed a modest arsenal of drugs able to counteract some of the most common mutations.

Yet many patients learn that their cancers have mutations for which no drug exists. In fact, the roles many of these genetic changes play in cancer growth are...