Cancer is a disease of faulty DNA, I had been told. So this summer I went on a pilgrimage that many journalists writing about cancer make at some point in their careers: I took the train from London to the outskirts of Cambridge to visit the Wellcome Sanger Institute. It is a storied place for discoveries about how genes shape our health. Back in the 1990s Sanger scientists decoded a third of the first fully sequenced human genome. Since then their research has led to drugs for cancers that previously had no treatment at all.

What I learned there was astounding. The glitches in our DNA known as “cancer-driver” mutations may not be that at all. Scientists thought they were villains because they consistently turn up in tumours. But now they are finding them in perfectly healthy tissues, too. It is a fascinating story about the origins of cancer—and what it might take to prevent it.