Comparing the effort to unravel all human genes to the push a generation ago that put men on the moon, government leaders yesterday celebrated a milestone in the project: the decoding of 1 billion units of genetic information, roughly a third of the total.

When the project is done, no later than 2003, "we will better understand the miracle of how a single cell develops into a unique human being," Donna Shalala, the secretary of health and human services, said in a speech praising the thousands of researchers around the world who are part of the effort. "We will better understand the history of life on earth. And we will better understand what binds us all together as human beings."

Managers of the Human Genome Project, as the international collaboration is known, called a quick celebration at the National Academy of Sciences yesterday to mark a milestone that was actually reached on Nov. 17. On that day, the billionth unit, or base pair, of human genetic information was deposited in GenBank, the giant Internet database that holds the results of gene research.

Because the entire human genetic code is estimated to contain about 3.2 billion base pairs, researchers believe they're about a third of the way toward finishing a draft gene map. Francis Collins, director of the National Human Genome Research Institute, said the draft should be completed by next spring, with two to three years of polishing to follow.

Yesterday's celebration emphasized the strong international flavor of the work, which is being done by 16 laboratories in the United States, Britain, France, Germany, Japan and China. The effort, expected to cost at least $3 billion by the time it is complete, is being supported in this country by the Department of Health and Human Services and the Department of Energy.

Researchers joked yesterday about the difficulty of explaining their work. In one sense, they are simply reading the order of letters in a highly repetitive template that regulates human biology. There are only four letters: A, T, C and G, representing the chemicals adenine, thymine, cytosine and guanine. They appear in monotonous strings that look like GAATTCCATA CATTTCTTCT, and go on and on for billions of letters.

But the order of those letters is the key to many of the great mysteries of life. Every human cell has a copy of the master template, and the particulars of genetic sequence determine how that cell behaves. The Human Genome Project promises to shed light on such puzzles as how people respond to infectious disease, why some people get cancer and others don't, how arteries in the heart clog up, how babies develop and what can go wrong in the process, and why people grow old and die.

The Human Genome Project was on a leisurely course for completion in 2005 when its leaders were shocked last year by a series of developments. Several private firms, benefiting from technology developed through years of public investment in the project, announced large, rapid gene-sequencing ventures. Most notably, Celera Genomics Corp. of Rockville said it would unravel the entire human genome faster than the Human Genome Project.

The competition sparked a notable acceleration in the public venture. The goal of a rough draft is new, and the final deadline was moved up two years. Collins said yesterday that most of the billion base pairs now in GenBank were unraveled in the past seven months, indicating that the project is progressing rapidly.

Several institutions were praised yesterday for making particularly large contributions to the project. They included gene-sequencing units at the Baylor College of Medicine in Houston, at Washington University in St. Louis, and at the Massachusetts Institute of Technology in Cambridge, Mass.; a Department of Energy unit called the Joint Genome Institute in Walnut Creek, Calif.; and the Sanger Centre, a huge sequencing center in Cambridge, England.