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Yet the criminal case uses of DNA fingerprinting were not the first to occupy Jeffreys and his team. Its usefulness in immigration cases grabbed immediate attention. A paper about DNA fingerprinting was written by Jeffreys and his team and was published in Nature in March 1985, triggering several newspaper reports. These were instantly followed up by a group of lawyers who were fighting the deportation of a young boy who, said the Home Office, was not the son of a British woman, as she claimed, and had no right to UK nationality.
"In fact, I had never seen the implications for immigration cases," admits Jeffreys. "It was my wife, Sue, who said DNA fingerprinting would make an incredible difference in disputes over nationality. And she was absolutely right."
After talks with the woman's lawyers, Jeffreys agreed to help. However, the case was complicated by the fact that the boy's father was no longer living in Britain and could not be contacted. "It was like a jigsaw with most of the bits missing," says Jeffreys.
Nevertheless he took samples from the mother, her three daughters and the disputed son. The results "blew me away", he recalls. "It was so incredibly simple. When I looked at the film we made of the DNA samples, I could see that every genetic character in the boy was either present in the woman or in his sisters. He was definitely her son."
The Home Office called in Jeffreys and, after a detailed explanation by him, agreed to drop the case. "Afterwards I went over to tell the mother what had happened, that DNA had done its job," says Jeffreys. "She had a bad time for the past two years and it was clearly affecting her health. But the look on her face when I told her, the relief - it was a magical moment. I realised then that we were on to something of real use. We had reached out and touched someone's life."
Over the next decade, DNA fingerprinting was used to test more than 18,000 immigrants who had been refused entry into the UK. Of these, more than 95% produced results that showed they were blood relatives of UK citizens and were therefore entitled to British citizenship - thanks to DNA fingerprinting.