It was the eureka moment for which detectives investigating the Claremont serial killings had waited over a decade.

In January 2009, a crucial link was made between the apparent murders of three young women after they vanished from the streets of the Claremont entertainment precinct in 1996 and 1997 and the brutal rape of a teenage girl in 1995 as she walked home from Claremont at night.

The disappearances of receptionist Sarah Spiers, 18, childcare assistant Jane Rimmer, 23, and 27-year-old lawyer Ciara Glennon had shocked Perth, even more so when the bodies of Ms Rimmer and Ms Glennon were found in bushland weeks after they were last seen.

But despite unprecedented publicity and police appeals for help, there had been no arrest.

While little appeared to be happening publicly, behind the scenes the Macro Task Force — set up specifically to investigate the case — was working diligently to find the killer.

In June 2008 then-deputy police commissioner Chris Dawson ordered a review of potential lines of inquiry, asking his team to pursue testing of the forensic evidence overseas, where more advanced techniques were being used.

Police forensic expert Sergeant George Paton, together with Laurie Webb, a senior scientist at PathWest, the state's pathology laboratory, were tasked with taking key samples that had been retrieved from Ms Glennon's body to the UK for testing.

Ciara Glennon's body was found in bushland in Perth's northern suburbs. ( ABC News )

More viable samples could be extracted from Ms Glennon's body than from Ms Rimmer's, as it had been discovered 19 days after her disappearance. In contrast, Ms Rimmer's body had been exposed to the elements for 55 days before being found.

Nonetheless, both bodies were significantly decomposed and much of the important forensic material police might expect to find at a crime scene was no longer present.

DNA technique used in Falconio murder

By 2008, various samples from the bodies of Ms Glennon and Ms Rimmer had already been subjected to a variety of forensic testing, both in Australia and New Zealand, but nothing of note had been discovered and detectives were desperate for a breakthrough in the long-running case.

The UK testing — using a highly specialised technique unavailable in Australia at the time that amplifies the tiniest fragments of tissue samples — proved pivotal.

Low Copy Number (LCN) testing had been used in the UK for a number of years by then and had been employed to solve cold case murders and other serious crimes within Britain and elsewhere.

This included the murder of young British backpacker Peter Falconio in Barrow Creek, in the Northern Territory, in July 2001.

DNA evidence helped convict Bradley Murdoch of Peter Falconio's murder. ( AAP )

His killer, Bradley Murdoch, was arrested because his DNA was found on three crucial exhibits — the t-shirt Mr Falconio's girlfriend Joanne Lees was wearing on the night he attacked the pair, the gearstick from the couple's Kombi van and the makeshift handcuffs he used to bind her hands together.

Initially only the t-shirt had yielded traces of Murdoch's DNA, but a closer examination of samples from the gearstick and handcuffs by Jonathan Whitaker of the UK's Forensic Science Service, using the LCN technique, found microscopic DNA samples that matched Murdoch.

Joanne Lees with her boyfriend Peter Falconio, who was murdered by Bradley Murdoch in 2001. ( AAP )

But the LCN technique has not been without controversy.

In 2007, the year before the Claremont samples were sent to the UK, use of LCN was temporarily suspended by British authorities following the Omagh bombings trial, in which Sean Hoey was acquitted of the murders of 29 people in Northern Ireland in 1998.

The judge in that trial had concluded that LCN testing — on which much of the Omagh evidence centred — was unreliable, but the suspension was lifted after an investigation found it to be a sound technique for detecting DNA.

Karrakatta rape link discovered

Four samples from Ms Glennon's fingernails were taken to the UK, along with eyelash and hair samples.

In the UK lab, a decision was made to combine two of the samples, known as AJM 40 and AJM 42, which comprised scrapings from Ms Glennon's left thumbnail and middle fingernail.

Once this combined sample was tested, a breakthrough finally came.

DNA from an unknown male was discovered on the sample by Dr Whitaker, the same DNA expert involved in the Falconio case.

Bradley Edwards does not contest that his DNA was found on samples from Ciara Glennon's fingernail scrapings. ( Supplied: Supreme Court of WA )

Details of this tiny DNA sample were entered into both the UK and Australian databases, but no match was found.

But on January 16, 2009, the eureka moment came.

Having returned from London, Laurie Webb entered the unknown man's DNA onto the WA database — and there was a match.

The sample matched DNA extracted from intimate swabs taken from a 17-year-old girl raped in February 1995 at Karrakatta Cemetery, leading police to believe the same perpetrator was responsible for murdering Ms Glennon.

It would be close to another eight years before police were able to identify their suspect — Telstra technician Bradley Robert Edwards — through a convoluted trail of evidence from a series of crimes he committed as early as 1988.

These included the Karrakatta Cemetery rape, to which Edwards admitted just weeks before his murder trial was due to start

Edwards has admitted raping a 17-year-old girl at Karrakatta Cemetery in 1995 after abducting her as she walked home from Claremont at night. ( ABC: Emma Wynne )

But this vital DNA breakthrough that prosecutors argue links the rape with the murders came under the spotlight this week in the WA Supreme Court and is expected to be the focus of the trial in the coming weeks.

The crucial PathWest testimony

The evidence of PathWest staff — which began on Thursday when forensic scientist Anna-Marie Ashley took the stand — will be pivotal to the case against Edwards.

Ms Ashley outlined in painstaking detail the way PathWest received samples from both the Karrakatta rape victim and Ms Glennon's post-mortem examination, and the administrative processes involved in labelling and documenting the items, storing them, extracting sub-samples from the main samples and analysing them.

Every step of the process has been laid bare, underscoring the importance of this evidence to the case.

Samples from the rape victim were held in the same laboratory and analysed by the same PathWest scientists as the Glennon samples, and the possibility of cross-contamination between the samples is something the defence has already flagged as a major focus.

Defence counsel Paul Yovich SC said in his opening address, "exhibit handling, recording and forensic processing protocols were not always satisfactory".

While Edwards has conceded it is his DNA on Ms Glennon's fingernail samples, Mr Yovich has indicated he would try to cast reasonable doubt on how it got there.

In his opening address, he questioned whether the DNA came from her fingernails, "or perhaps from the sides of the yellow-top containers in which that debris had been stored, or perhaps from a combination of some or all of these things".

State prosecutor Carmel Barbagallo SC, in contrast, has stated firmly that "DNA within a particular sample doesn't just fly around a laboratory".

Key parts of the defence's argument are expected to be revealed when Mr Yovich begins what is expected to be a robust line of questioning of Ms Ashley on Monday.