All of us have a most remarkable gene lurking in our DNA. Its job is to protect us from cancer. This gene, known simply as p53, constantly scans our cells to ensure that when they grow and divide as part of the routine maintenance of our bodies, they do so without mishap. If a cell makes a mistake in copying its DNA during the process, p53 repairs it before allowing the cell to carry on dividing. If the mistake is irreparable and the rogue cell threatens to grow out of control (as happens in cancer), p53 commands the cell to commit suicide. Cancer cannot develop unless p53 itself is...