In the scientific community, its revolutionary significance has been compared—and safely so—with man's first step on the moon. On June 26, biologists J. Craig Venter and Francis S. Collins announced that their two research groups had mapped the human genome: a strand of DNA whose 3 billion chemical parts spell out the genetic code for what makes up a human being. The breakthrough opens the door to decades of discovery, notably in the early detection and treatment of diseases ranging from simple allergies to the most lethal cancers.

The 10-year effort to map the genome came down to a bitter race between Venter, 53, CEO of Celera Genomics, a private Rockville, Md., lab, and Collins, 50, director of the government's publicly funded National Human Genome Research Institute. Only after White House intervention did the two rivals come together and declare their contest a draw. "It was exciting because scientists took what was thought to be an impossible task and completed it earlier than anyone would have thought," says David R. Cox, 53, a pediatrician and codirector of the Stanford Human Genome Mapping Center, one of 15 other groups that have also been working on deciphering the code. Cox, who lives in Belmont, Calif., with his wife, Vicki, a genetics counselor at the University of California at San Francisco, and their three children, spoke with national correspondent Giovanna Breu.

What is a genome?

Inside the nucleus of every one of the billions of cells that make up a human being is a genome, or DNA blueprint, for that individual. It can be thought of as a necklace made up of 3 billion pearls in four different colors—chemicals abbreviated A, G, T and C for adenine, guanine, thymine and cytosine. Every organism has a genome defined by the number and the pattern of those four chemicals.

How does a genome differ from a gene?

The genome "necklace" is broken up into 23 segments, called chromosomes. A gene is a specific stretch of chemicals on a chromosome that directs the production of a particular protein. Proteins are the building blocks of all human functions and of characteristics like eye color.

How do you map the genome?

You have to figure out whether it's an A, G, T or C in each of the 3 billion positions on the necklace. Three billion things are not too much for genetic sequencing machines to handle, but you can only look at a string of about 500 at a time. That is why it has taken so long.

What can we learn from this map?

It serves as a foundation in understanding the fundamentals of biology and the complexity of life. By comprehending how biology works, we can find drugs that will treat disease. Instead of guessing that oat bran keeps you from getting colon cancer, you actually will understand the biology and know what keeps you from getting colon cancer. It will play an important role in our understanding of a number of diseases, but we will not be able to conquer all of them.

Why not? For example, gene therapy recently restored the immune systems in two French babies born with "bubble boy disease."

That happened to be a rare disease where one gene was not working right and they replaced it. Other conditions such as cystic fibrosis and sickle-cell disease are also caused by one mutated gene, and they may be manageable. But most diseases are not going to work that way. They are caused by mutations in many genes, and that won't be so easy. Still, we hope to become able to identify people with certain gene patterns that spell disease and then develop treatments that fit their individual genetic profile. People tend to look at genetics simplistically, as if by knowing the structure of the genome, we'll be able to shape it at will. If that day comes, it's in the remote future. Even a single human trait, say hair color, derives from a large number of genes and proteins interacting in ways we haven't begun to understand.

Should information gained from the genome be owned by the public or, as Venter argues, by the companies who discover it?

The issue is whether the human genome sequence will be publicly available or whether one company can be in control of it and sell it. Dr. Collins and members of his group frequently say that Dr. Venter will not make his genome sequence available for people to use. On the other hand, Dr. Venter says he will, but he will package the information and charge people for it. I would argue that in the long run having scientists or industry pay a fee for the raw information is not in society's best interest. Still, it is standard practice to pay licensing fees when a product is created.

If our makeup is made public, couldn't someone discriminate against us because of a genetic predisposition to a disease?

Genetics does run the risk of being used falsely, or carelessly, to discriminate against people. In fact, however, information in the genome could be misleading. Just because you have genes that appear to be related to getting cancer doesn't mean you will necessarily develop cancer.

Some worry that mapping the genome allows us to play God by manipulating life.

The genome is likely to give an insight into how to manage life, but again, it is not going to allow us to design a perfect human being. If through genetics I could be sure that my kid was going to be completely happy, the smartest person in the world, love life and never be anxious, I would do it in a second. But we just have the parts, not the instruction manual. I think God isn't so stupid as to let anyone have that. Even if we ever do decipher it, I think human life is far too complex for us to create designer people.

What will the future bring?

The genome gives us a list of what living things are made up of, but not how they go together and work. It provides one more piece of information that we can start using to make order out of our ignorance and help people to make better decisions in life. But exactly what its use will be in biology remains to be discovered. After all, when Edison invented the phonograph, he never imagined it mainly would be used to play music.