A stain suggests the man accused of the World's End murders had sex with victim Helen Scott, a trial has heard.
Helen Scott and Christine Eadie went missing in 1977
DNA tests have been used on the lining of 17-year-old Ms Scott's coat since she and her friend Christine Eadie, 17, were found dead almost 30 years ago.
The High Court in Edinburgh heard how semen closely matching swabs from Angus Sinclair was found mixed with cells with the same DNA profile as Ms Scott.
Mr Sinclair, 62, denies raping and killing the teenagers in 1977.
The bodies of the two girls were found after a night out at Edinburgh's World's End pub.
Angus Sinclair, denies acting with Gordon Hamilton, who has since died, in their rape and murder.
Forensic scientist Dr Jonathan Whitaker told the trial: "I have calculated that a fair and reasonable estimate of obtaining this profile if the semen had originated from someone other than, and unrelated to, Angus Sinclair is approximately one in a billion.
"This result would therefore provide extremely strong support for the assertion that the semen detected on the coat of Helen Scott has originated from Angus Sinclair."
The trial has heard how brothers and sisters of Mr Sinclair's dead brother-in-law, Gordon Hamilton, provided samples for DNA testing.
The results were compared with DNA found in semen in the bodies of the victims.
Dr Whitaker said the results were what he would expect if the semen came from a brother of the surviving Hamiltons.
The girls were last seen alive at the World's End pub
The semen DNA was also compared with tiny amounts of genetic material found on ceiling coving taken from a flat in Dennistoun, Glasgow, where Hamilton used to live. Again there was a match.
Dr Whitaker said taken together, the results provided "compelling evidence" that the semen in Ms Scott and Ms Eadie came from Hamilton.
The Crown claim both men were involved in the double rape and murder.
Mr Sinclair claims any sexual contact with the girls was with their consent and that Mr Hamilton was the killer.
It is alleged that on the night of 15-16 October 1977 Mr Sinclair and Mr Hamilton persuaded or forced the girls into a motor vehicle, in Edinburgh's St Mary's Street or High Street, near the World's End pub.
They were then, it is alleged, attacked, stripped, tied up and strangled. The bodies were found dumped in East Lothian, about seven miles apart.
Prosecutors have finished their case. There will now be legal debate in the absence of jurors, who have been told not to return to the High Court until Monday when the trial will continue.