A 42-year-old man has been jailed for six years after being traced by police through the DNA of a relative.
Daniel Alderson, of Waterlooville, Hampshire, admitted two indecent assaults dating back to 1992 and 1997.
Police used a new technique which finds close matches to DNA specimens found at crime scenes, enabling officers to identify close relatives of offenders.
The sentencing has been heralded by the police as a breakthrough in sex crime detection.
Det Sgt Doug Utting warned that criminals now have to make sure their relatives do not get into trouble with the police if they want to avoid being caught for old crimes.
He said: "If an individual thinks that as long as they do not get into trouble again that they will be all right, they are wrong.
"Now they would have to make sure their father, brother, sons, etc, stay away from the police because otherwise there is the potential they will get caught
Alderson, of Rudolph Court, was sentenced on Friday to four years' imprisonment for the indecent assault of an 18-year-old woman on 30 August, 1992.
He was also sentenced at Portsmouth Crown Court to six years to run concurrently for the indecent assault of a 12-year-old girl on 6 March, 1997.
The first offence happened in a subway in Langstone,
The second offence took place in a subway in Portsmouth.
In August 2000, the first breakthrough in the case came when the Forensic Science Service found DNA samples from both attacks matched.
But it was not until four years later that new technology could be used to carry out a familial DNA search on the national database.
Alderson was arrested on 18 August 2004 after the search found a similar sample from a man who had been arrested for a minor criminal offence.
Alderson provided a DNA sample which gave a one-in-a-billion match to the specimens from the crime scenes, the court heard.
Although he refused to comment in police interview, Alderson, a machine operator, went on to plead guilty to both offences at court on 23 September this year.
Alderson was ordered to sign on the sex offenders' register for the rest of his life.