Who should be tested first when a mutation runs in a family?

When possible, it is generally recommended that genetic testing for a hereditary cancer syndrome begin with a family member who has had cancer. This is a decision to make with a care team, and this page is educational rather than advice.

The reason, explained by the National Cancer Institute, is that testing an affected relative first gives the clearest information about whether a specific change is actually present in the family. That makes everyone else's results easier to interpret.

For example, once a family's change is known, a relative who tests negative for it has a true negative — meaning they did not inherit that change and their risk is about the general-population level. Without a known family change, a negative result is harder to interpret. A genetic counselor can help a family decide who to test first and in what order.