Researchers at the American Cancer Society have only to look back half a century ? when a genetic discovery of comparable magnitude rocked the scientific community ? to predict how human genome mapping will affect future cancer research.

In 1953, James Watson, Ph.D., (a former American Cancer Society grantee) and Francis Crick, M.D., established the double helical structure of DNA. Answers to questions that plagued scientists and doctors throughout the 19^th century and the first half of the 20^th century could finally be addressed, now that they knew the precise nature of DNA.

Watson and Crick received the Nobel Prize in 1962 for this groundbreaking achievement.

DNA contains the genetic code that gives orders to all human cells. After the Watson-Crick discovery, scientists learned to translate this code and to better understand how genes work and how genes could be damaged by mutations (changes or mistakes in genes).

These modern techniques of chemistry and biology answered many complex questions about cancer. Scientists already knew cancer could be caused by chemicals, radiation and viruses, and that sometimes cancer seemed to run in families. But, as their understanding of DNA and genes increased, it became apparent that it was the damage to DNA by chemicals and radiation or introduction of new DNA sequences by viruses that often led to the development of cancer.

Pinpointing the exact site of the damage to a specific gene in the DNA became possible. Also, scientists began to discover that sometimes defective genes are inherited and that these inherited genes are sometimes defective at the same points that chemicals exert their effect. In other words, most carcinogens cause DNA damage (mutations), mutations lead to abnormal groups of cells (called clones), mutant clones evolve to ever more malignant clones over time, and the cancer progresses by more and more genetic damage and mutations.

Normal cells with damaged DNA die; cancer cells with damaged DNA do not. The very recent discovery of this critical difference answers many scientific questions that have troubled scientists for many years.

Today, the study of cancer biology has become a very complex science as, slowly, medical scientists are identifying genes that are damaged by chemicals or radiation and the genes that, when inherited, can lead to cancer.

The recent discovery of two genes that cause some breast cancers, BRCA1 and BRCA2, represents considerable promise because many individuals who have a higher probability of developing breast cancer can now be identified.

Other genes have been discovered that are associated with some cancers that run in families such as cancers of the colon, rectum, kidney, ovary, esophagus, lymph nodes, skin melanoma and pancreas.

Familial cancer is not nearly as common as spontaneous cancer, causing less than 15 percent of all cancers, but it is important to understand these cancers because scientists may be able to identify persons at very high risk.

Experts anticipate discoveries of even more cancer susceptibility genes will be among the earliest prizes to be reaped by the recent human genome sequencing. Gene-based therapies to treat cancer ? and use of genetic information to prevent cancer via vaccines, for instance, will come later.