The battle over DNA evidence continued in the Candance Derksen murder trial on Friday morning, with the defence attacking the credibility of a DNA expert and the work of his lab.

The morning began with testimony from Curtis Hildebrandt, a scientist with a company that Winnipeg police used to test DNA evidence in the investigation.

Mark Edward Grant faces a charge for first-degree murder in the case, which dates back to November 1984.

He has pleaded not guilty.

Derksen, 13, disappeared while walking home from school in November of 1984. Seven weeks later, her body was discovered bound and frozen in a shed in Elmwood.

The Crown has previously argued Grant's DNA was found on the twine used to bind Derksen and a number of his hairs were found at the scene.

On Friday, Hildebrandt testified that Grant's DNA profile matched those of seven hairs found on Derksen's clothes and near her body.

In cross examination, defence lawyer Saul Simmonds questioned Hildebrandt extensively about his credentials and the processes used in his lab.

Simmonds charged that many of Hildebrandt's publications were not peer-reviewed and that the bulk of his DNA testing experience concerns paternity cases, rather than criminal cases.

The slides that Hildebrandt obtained the crime scene hair samples from were damaged, and Simmonds asked him repeatedly if that happened while they were in his possession.

Hildebrant said he couldn't recall if he received them that way or not, but said he did not disturb the slides.

Simmonds has questioned witnesses repeatedly about the possibility of contamination and the way the evidence was preserved between 1985 and the mid-2000s when it was re-examined for DNA evidence.

Simmonds also questioned Hildebrandt about an error in his lab's procedures. Hildebrandt testified that he learned, months after the Derksen evidence was examined, that the process he was using was actually comparing a slightly smaller section of the DNA than meets standard practices.

Hildebrandt said the lab's processes were later changed.

He testified that the report he gave to Winnipeg police was not corrected.

The trial is continuing in Winnipeg with multiple witnesses set to testify.