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

When possible, genetic testing for a hereditary cancer syndrome should begin with a family member who has had cancer. That is what is generally recommended. It is still a decision to make with a care team. This page is educational rather than advice.

The National Cancer Institute explains why. Testing an affected relative first is the clearest way to find out whether a specific change is present in the family. That makes everyone else's results easier to interpret.

For example, say a family's change is known. A relative who tests negative for it then has a true negative. That means they did not inherit that change. Their risk is about the same as the general population's. 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.