WHEN Nidal Ayyad sealed the envelope, the act helped seal his fate.
A forensic analysis of saliva on an anonymous letter to the New York Times after the World Trade Center bombing last year linked Mr. Ayyad to the terrorist attack. The saliva test was only a small part of the evidence that eventually led to the conviction of Ayyad and three other men for the New York City bombing, but it illustrates the vast crime-solving potential of genetic biology.
From the envelope flap, Federal Bureau of Investigation laboratory examiners extracted a sample of Ayyad's DNA - deoxyribonucleic acid, an organic substance in cells. Each person (except identical twins) has a unique DNA makeup. Matching the DNA sample from the envelope with a sample obtained from Ayyad, law-enforcement officials identified him as the writer who claimed credit for the bombing.
Because blood and semen are the DNA-bearing cellular materials most commonly found at crime scenes or on crime victims, DNA testing is often used in cases of murder, rape, and other violent crimes. But it has wider forensic applications as well. ``Even a burglar who scratches himself breaking a window leaves behind a DNA calling card,'' says John Hicks, director of the FBI's laboratory division in Washington.
Thanks to a recent advance in DNA technology called polymerase chain reaction (PCR), which lets researchers reproduce the DNA in minute samples found at crime scenes, scientists ``can get very probative information from DNA samples that formerly were just too small to examine, like saliva from cigarette butts or the back of postage stamps, and even the DNA in the root of a single human hair,'' says Paul Ferrara, director of the state of Virginia's Division of Forensic Science.
Dr. Ferrara calls DNA analysis ``a profound development in forensic science, even greater for identifying individuals than the use of fingerprints starting in the last century.''
DNA testing has great implications for exonerating criminal suspects as well as convicting them. According to Ferrara, about 25 percent of the tests his department performs absolve suspects. Barry Scheck, a professor at Cardozo Law School in New York and a criminal-defense lawyer, heads up an ``Innocence Project'' at the law school. ``We have used DNA evidence to get a lot of people out of jail, including a prisoner on death row,'' Mr. Scheck says.
The crime bill in Congress includes a section on DNA testing. It would standardize testing procedures, establish quality-control standards for labs, provide grants for state and local forensic labs to develop DNA testing capability, and authorize a national databank for DNA samples.
DNA testing procedures are too complex to explain in a short article. In summary, after several steps to extract DNA from cells, slice it into fragments of various lengths, and arrange the fragments in patterns, examiners can display the patterns on X-ray film in lanes that resemble crude supermarket bar codes. They can then compare patterns of DNA from a crime scene or victim with DNA obtained from a suspect or located in a DNA databank, in search of a match, or ``hit.''
This analysis of cell fragments to single out criminal suspects is sometimes called ``DNA fingerprinting.'' Forensic experts prefer the terms DNA profiling or identification, however. For fingering suspects, DNA analysis has the potential to be more effective than actual fingerprints; but also, as currently obtained, DNA evidence has certain limitations less applicable to fingerprints:
* Although performing forensic DNA tests isn't akin to rocket science, it requires technical precision by skillful examiners. Mistakes occur, both in the chemical procedures and through such human errors as mislabeling vials containing the DNA samples.
* While each person's complete DNA profile is, unique, the small segments of a person's DNA chain capable of being forensically examined are not necessarily unique. Certain DNA patterns recur in humans' genetic makeup, with degrees of frequency determined partly by such factors as race, ethnicity, or inbreeding among groups.
Consequently, once they have matched a suspect's DNA with a DNA sample from a crime scene, forensic examiners, drawing on data from wide population samples, must calculate the probable frequency with which the identified DNA pattern would crop up in different humans. Thus, an examiner at a trial might testify that a suspect's DNA profile would be repeated in 1 in a million people, or 1 in 100,000, or 1 in just several hundred.
A debate has raged in the scientific and criminal-justice communities over the representativeness of the population samples used to make such estimates and the proper methods of calculation.
Defense lawyers have seized on both factors - possible testing errors and the dispute over genetic population studies - to challenge DNA evidence in court.
According to Ferrara, the debate over population statistics - that is, the significance of getting a DNA match - is dying down. The toughest challenges from defense lawyers, in his experience, are on the accuracy of specific DNA tests and, more broadly, on the adequacy of DNA quality-assurance standards and safeguards in forensic labs.
Professor Scheck and New York City attorney Peter Neufeld, the co-chairmen of the National Association of Criminal Defense Lawyers' DNA task force, assert that quality-assurance standards are lax in many forensic labs that perform genetic testing. They call for more ``blind'' (disguised) proficiency testing of labs' DNA procedures by outside examiners, and they argue that labs' error rates should be calculated and presented to juries.
Many forensic experts contend that the federal crime bill would address most of the concerns about DNA testing standards and proficiency monitoring, however.
Not even critics like Scheck and Mr. Neufeld disparage the underlying DNA science (indeed, they praise its application to freeing innocent people). In 1991, a committee of genetic scientists and forensic experts established by the National Research Council, National Academy of Sciences, reported after a two-year study that DNA testing ``is a fundamentally reliable technology when properly applied by qualified people,'' in the words of Ferrara, a member of the panel.
Clearly, DNA profiling is here to stay. In Ferrara's large and well-funded forensic department, DNA testing has totally replaced traditional serological tests (such as comparing blood types), and in short order most other labs will catch up, especially if federal grants are forthcoming, he says.
Within five years, he predicts, law-enforcement officials will use and exchange DNA evidence as routinely as they utilize fingerprints today.