When the war on cancer was declared by the United States Congress in 1971, optimism was high that a magic bullet could be found to cure the cluster of diseases collectively known as cancer. Now, 26 years later, the dream of discovering a single antidote has long since been abandoned. Instead what began more than two and one-half decades ago has led researchers to an understanding of the complex workings of cancerous cells and what goes wrong in normal cells to cause the ominous changes that lead to the disease.