Explained: What makes some of the mutations cancerous, but not all

Healthy cells carry a surprising number of cancer-linked mutations, but they don't turn into tumors. What's holding them back?

Cancer is a disease of mutations. Tumor cells are riddled with genetic mutations not found in healthy cells. Scientists estimate that it takes five to 10 key mutations for a healthy cell to become cancerous.

Some of these mutations can be caused by assaults from the environment, such as ultraviolet rays and cigarette smoke. Others arise from harmful molecules produced by the cells themselves. In recent years, researchers have begun taking a closer look at these mutations, to try to understand how they arise in healthy cells, and what causes these cells to later erupt into full-blown cancer.

The research has produced