UNTIL ABOUT TWENTY years ago, cancer was a "black box." No one knew what happened to make a cell become cancerous. Researchers had no idea why some tumors grew slowly and remained in one place while others spread through the body like a forest fire. Today, however, scientists know that all cancers start because of changes in genes.

The cell's computer programs

Genes are the "computer programs" that tell each cell in the body what to do throughout a person's life. A human's 100,000 or so genes are carried in twenty-three pairs of wormlike bodies called chromosomes in the central body, or nucleus, of the cell. Each gene is part of a long, complex molecule of a chemical called deoxyribonucleic acid, or DNA. Each gene usually contains instructions for making one chemical in the cell. The gene's message is encoded in the arrangement...