Illustration: David Plunkert

On 4 January 1998, police in London arrested a man, whom court records call “B,” on suspicion of burglary. The police swabbed the inside of the suspect's cheek to collect a sample of his DNA.

In August, B was acquitted and released. But in September, B's DNA profile was—accidentally and illegally—entered into the United Kingdom's national DNA database. The system automatically compares newly loaded DNA profiles against unidentified samples obtained from crime scenes. The system found a match—a sample recovered from a 1997 rape and assault case. The police arrested B, and the government successfully prosecuted him for those crimes.

Is there anything wrong with such a turn of events? Privacy advocates say there is, as do people worried about racial discrimination. Among these are lawyers working with the American Civil Liberties Union (ACLU) and the Council for Responsible Genetics, in the United States, and with GeneWatch and Privacy International, in the United Kingdom. Law-enforcement officials and forensic scientists, on the other hand, say the use of such a tool is invaluable for solving crimes, not only to match evidence from a recent crime to an individual in the database but also to link some unsolved cases, showing that they share an as-yet-unknown perpetrator.

Since that 1998 incident, governments have been rapidly expanding the collection of DNA for databases, and changes in database-searching technology that target near matches are raising new concerns. As a result, civil libertarians and privacy advocates are lobbying for restrictions, while some scholars are pushing in the opposite direction, arguing that the only fair way of building a DNA database is to create a universal one—that is, to record the genetic profile of each citizen.

The information loaded into such databases reflects a feature of DNA known as short tandem repeats (STRs). DNA contains a sequence of paired bases, or nucleotides, of which there are four types. The human genome contains about 3 billion such base pairs, arranged into 23 pairs of chromosomes. A small subset of the long sequence creates the 20 000 or so human genes, most of which code for the proteins that determine a person's biochemical makeup and physical characteristics. The rest—about 98 percent—is noncoding DNA. Although scientists are discovering that a surprisingly high fraction of these seemingly useless sequences may affect the body's functions, some of them seem clearly to be meaningless artifacts of evolution.

In certain sections of the human genome, the noncoding DNA contains repeated patterns of two to five nucleotides, the number of repeats in each sequence varying by person. For forensic typing, scientists consider repeats at several loci, or positions on the genome. The number of repeats at each locus is known as an allele. People have two alleles at each locus, one from each parent, that vary in length depending on the number of repeats.

In the United States, the Combined DNA Index System (CODIS), established by the FBI in 1990 to link existing local, state, and federal systems, is based on STRs at 13 loci. In London, the Home Office currently relies on STRs at 10 loci. Although the estimated rarity is different for each DNA profile, the estimated rarities of complete profiles can be smaller than one in a trillion.