passenger mutation

If a driver mutation is the one steering the tumor, passenger mutations are the riders along for the trip who do not touch the wheel. A cancer cell can carry many mutations simply because it divides so much and its DNA repair is faulty; most of these changes do nothing to help the tumor grow.

Technically, a passenger mutation is a change that does not provide a selective growth advantage. It often arises in the same cell as a driver and then gets carried into all the cell's descendants as the tumor expands, hitching a ride on the driver's success without contributing to it.

Passengers vastly outnumber drivers, which makes finding the true drivers a needle-in-a-haystack problem. They are not always useless to scientists, though: their patterns can reveal a tumor's history and which DNA-damaging processes have been at work, like footprints showing where the cell has been.