DNA matching that of the man accused of strangling Elaine Doyle was found on her naked body, but not on her clothes.
Forensic scientist Pauline McSorley told a trial on the 28th anniversary of the Greenock teenager's death, it was more than a billion times more likely that the DNA profile she found came from John Docherty than from any other unrelated male.
She agreed that sweat or droplets of saliva deposited as a perpetrator strangled the 16-year-old girl could account for her findings. Mrs McSorley, 57, described years of work on samples taken from the crime scene.
She and her colleagues examined tape which had been laid across Ms Doyle's body in the hope of trapping hairs or fibres which might help track down her killer.
The forensic scientists also tested the teenager's clothes for clues.
The High Court in Edinburgh heard yesterday how some tiny traces matched the DNA of police who had handled clothing or the body, forensic scientists, and a member of the laboratory staff. But two results could not be accounted for until Mr Docherty volunteered a sample of his DNA in May 2012 and gave another sample on March last year, when he was arrested and charged with the murder.
The trial heard that the DNA on Ms Doyle's back was an exact match.
DNA on the girl's face also matched Mr Docherty's profile. Mrs McSorley said it was 560,000 times more likely it came from the accused than any other unrelated male.
She told the trial that if the male DNA on Ms Doyle's back had got there as she socialised at a disco in Greenock's Celtic Club or later in the town centre, she would have expected to recover DNA from the blouse, but didn't.
Mr Docherty, 49, now of Hunters' Quay, Holiday Village, Dunoon, denies murder. He claims at the time he is alleged to have stripped and strangled Elaine Doyle, 16, he was with his parents,who are no longer alive, at their home in Anne Street.