As recently as the middle of this century, cancer was still a mysterious disease. It seemed to strike with reckless abandon, and once it had gripped its victim, doctors could do little more than relieve the pain, steady the pulse and ease the breathing. It is all too easy now to reflect on this sad state of affairs without realizing that cell biology itself was also a rudimentary science.

In the past few years, a vastly different view of cancer has emerged. The highly sophisticated tools of genetic engineering have allowed biologists to look deep into the inner provinces of the...