KEY POINTS:
DNA samples helped police catch a homeless man who sexually assaulted two teenagers in inner-city Auckland parks last year.
A match between a saliva swab from one victim and a semen sample from the other sealed the convictions of Tapuatua Hiomai Onaariki, 20, for a string of sex offences, including rape.
Police were initially stumped after the first attack in August, when a 19-year-old university student was dragged into a dark part of Albert Park and sexually assaulted.
Before she fled, Onaariki kissed her with his tongue - leaving faint but valuable traces of DNA for police to track down.
Despite releasing an identikit picture of the offender, police had few leads until a second teenage girl was attacked in Myers Park five weeks later.
The 18-year-old secondary student from Northland had been drinking with a friend and walked alone to use a toilet in the middle of the park. Drunk and disoriented, she became lost - and then lost consciousness.
Onaariki found the teenager lying on a bench and dragged her to a more secluded part of the park where he raped her.
Detective Sergeant Andy King said DNA from semen from the rape matched a voluntary sample Onaariki had given to the police database.
The saliva from the Albert Park attack - which was more difficult to extract DNA from - also returned a positive match to Onaariki.
When arrested two weeks after the Myers Park rape, the homeless man admitted both attacks and explained his actions to police by saying he was "angry at women".
Onaariki later pleaded guilty to eight charges, including abduction, unlawful sexual connection and rape - which carries a maximum sentence of 20 years' imprisonment.
He was due to be sentenced in the Auckland District Court on Friday but the hearing was adjourned.
A chance DNA test last year caught a "cold case" rapist from a sample taken from a minor traffic incident.
Pop Tanupou E'Moala Aholelei admitted the 2001 rape of a woman in Auckland's Parnell Rose Gardens, an attack that was front-page news and sparked a major police inquiry.
But with few leads other than a DNA sample, the case was shelved.
Then in June 2006, Aholelei was interviewed by a constable after a car crash and - in what detectives describe as a naive decision - volunteered a DNA sample.
It was matched to the sample taken from the rape and Aholelei was arrested almost five years to the day later.
DNA PROFILING THE KEY
ESR principal scientist John Buckleton says standard DNA profiling would have been used on the incriminating saliva sample from the sexual assault victim's mouth.
In this method, scientists target 15 known sites of "junk DNA", bits of genetic material that don't code for any part of the body and vary hugely from person to person.
They can use a process Buckleton describes as a "molecular photocopier" to multiply a single cell millionfold to produce a workable sample.
This sample is then compared to a sample from the suspect, usually scraped from the inside of the mouth. Traces need only be infinitesimal, says Buckleton.
"It can be completely latent, which means it's invisible. We can genotype one cell."
In other cases, scientists have successfully profiled saliva off kissed breasts, and noseprints that peeping Toms have left on windows. It's not unusual for samples to contain DNA from sex offenders and their victims.
Tests can discriminate between two people's DNA material. Some techniques are male-specific, only working on male DNA.
Buckleton won't say how long another person's saliva can remain in your mouth and allow for DNA profiling.
"I don't want to educate criminals."
