DNA molecules provide the unique genetic codes that differentiate one person from another. Examiners are using the latest DNA technology in an ongoing effort to identify the more than 2,800 victims from the September 11 attacks at the World Trade Center.

The newest DNA company to officially join the huge project of identifying remains is Orchid Cellmark, which is based in Germantown, Maryland. The company's executive director, Mark Stolorow, says the DNA samples recovered from the World Trade Center have posed special challenges to researchers. "They are in the process of decomposing because of the high temperature and pressure of water that was poured onto that site for over three months while they were trying to put the fire out," he said.

Mr. Stolorow says Orchid Cellmark is using a technology that focuses on single nucleotide polymorphisms, or SNP technology, that he says has been used in molecular biology for nearly a decade, but is relatively new to forensics. He says SNP technology provides interpretable identifications using smaller pieces of DNA than the longer fragments that were required in other tests.

"We then modified our panel so that it was designed to test fragments of DNA that were only 60 to 80 nucleotides long. And that has proved to be very successful in determining interpretable DNA profiles from samples from Ground Zero," he said.

He refused to predict an overall success rate, but said the company is very enthusiastic about its findings from several batches of experimental samples.

Meanwhile, Kevin McElfresh, the senior vice president of one of the longest-serving DNA companies in the post-September 11 effort, Bode Technology Group, acknowledges that the results from the first round of testing were less than perfect. "Typically, you expect to get between 80-and-95-percent of the samples in a mass disaster to work, and I'm talking now, in the American Airlines flight that went down in Queens [New York] in November of last year, we actually did the bone work on that and we got about 90 percent of those samples to work. And, of course, you always want to get 100 percent, you know, we all start there, but reality is reality. In the case of the Trade Center, on our first pass through, we were getting about 50 percent and that just wasn't palatable to us," Dr. McElfresh said.

He says Bode has been able to raise its percentages of success by adapting and refining its DNA-extracting techniques. But both he and Mr. Stolorow say that some of the remains from the site were so badly burned or decomposed that it is impossible to identify them with any technology. "There are some samples that are literally just charcoal, and there's nothing to be gotten out of them. And that's about 10 percent of the samples, I would say, are just never going to work, Dr. McElfresh said.

He says another difficulty has been juggling the company's regular criminal justice forensics responsibilities with the huge volume of samples recovered from the World Trade Center. "How do you tell a rape victim that their case isn't important, even though the towers fell? Those people are important. Rape victims are important. Crime victims are important. And you have to just keep all those balls in the air at the same time," he said.

He said one thing Bode learned that will help it in future cases is how to handle and process large numbers of DNA samples.

For the survivors, though, identifying the remains may not be enough. Nikki Stern says she was not comforted by receiving the remains of her husband, Jim Portorti, who worked on the 96th floor of the first tower that was hit. "What was recovered, bluntly, was the size of a quarter (coin), and it hasn't provided any kind of closure for me," Ms. Stern siad.

But she says she has sent his remains to his parents, in the hopes that it will provide them with some sort of solace. "It's just never going to be enough for me and it's never going to make any sense to me," she said.

Out of more than 1,300 total identifications, the New York City Medical Examiner's office has so far identified more than 630 victims using DNA, alone.