Kind of strange, but the first use was to determine if children in the immigration system were in fact born to British parents. He thought that it could have many uses including to help law enforcement determine if someone had been at the scene of a crime.
The police from a small town in Britain had two separate rapes and murders of a couple of young girls. A 17 year-old boy with mental problems confessed to the crimes but the police didn't believe him. People were scared and young girls couldn't go anywhere without someone being with them and they certainly weren't allowed out at night.
One of the detectives heard about the new DNA technology and he called the Jeffreys to see if it could really work to determine who committed the murders. It hadn't been used up to this point so he was excited for the opportunity. He took DNA from both girls and then ran it against the young man who confessed and it clearly ruled him out.
They then did something we didn't do until years later as the ACLU was fighting against it, they took samples from all the men in the area and didn't get a match. Then someone came forward and said that a friend of his talked him into doing the DNA test for him; his buddy made up an excuse why he couldn't go. The police went to his home and arrested him and then when they took his DNA it was a perfect match. The detectives and Jeffreys were elated. It was groundbreaking. It took a long time for Jeffreys to explain in court why DNA was failproof and that no other person could've committed the crime other than the man on trial.
What Jeffreys did, changed the world. It is one of the greatest discoveries by mankind. Cold cases have been solved. Innocent people have been released from prison while the guilty parties are being locked up. Far less errors are taking place in ensuring the right person is charged with the crime. It has been a phenomenal deterrent as violent crime has declined in the U.S. by 51% since DNA testing became available. Amazing!