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If someone left a drop of blood at a crime scene, forensic scientists could analyze only the person's blood type plus a few proteins that exist in slightly different versions in different people.But neither of these tests is particularly specific: many people share blood types and protein markers, making unique identification from a blood stain nearly impossible.Advances in DNA (deoxyribonucleic acid) technology over the past 25 years have led to spectacularly precise forensic identification techniques, although some applications have also unleashed controversies regarding genetic privacy.Current molecular forensic work is pushing these technologies even further by analyzing extremely damaged DNA and by introducing RNA (ribonucleic acid) techniques to forensics.Most human forensic casework is performed with standardized commercial “multiplexes” that assay STRs at multiple genetic loci simultaneously.The ease with which STR profiles can be typed today has led to the development of large national databases containing STR profiles of millions of people suspected or convicted of crimes.Although the sequence is usually the same at each region in all people, the number of times that the sequence is repeated is highly variable among individuals.Jeffreys immediately saw the potential for forensic use of these markers, which he called “minisatellites.” In less than two years, forensic labs across the world could create DNA “fingerprints” of crime suspects by profiling their unique minisatellite makeup.

He denied, however, any connection to a three-year-old murder that police were convinced had been committed by the same person.

For the first time, forensic scientists could create genetic profiles so specific that the only people who share them are identical twins.

DNA fingerprint techniques evolved subtly over the next several years, until the polymerase chain reaction (PCR), developed by Kary Mullis, was introduced into forensic work.

STR profiles and databases The standard genetic forensic test used in crime labs across the world assays an individual's profile of markers, called short tandem repeats (STRs), which are genetic sequences similar to minisatellites, although the repeating DNA sequence in STRs is considerably shorter.

STRs are equally variable among individuals: with each additional STR locus a forensic scientist analyzes, the odds become vanishingly small that two people will have the same STRs at all loci.

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