LONDON, Ont. - The DNA testimony in Michael Rafferty's murder trial was attacked Thursday, with defence lawyer Dirk Derstine attempting to discount evidence linking victim Tori Stafford to Rafferty's car.

Derstine cross-examined forensic expert Jennifer McLean Thursday morning.

McLean agreed with Derstine there are a number of ways DNA can be transferred to an object. She also agreed that "sperm cell" DNA traces found in multiple sites in Rafferty's car did not necessarily come from semen.

There was only one sample from the car that was definitely linked to semen, McLean said.

Derstine then focused on the tiny spot of blood on a gym bag in the back seat that appeared to have mixed DNA from Stafford, Rafferty and other sources. Derstine presented McLean with a number of scenarios on how mixed DNA could have appeared on the bag.

Jurors heard Wednesday how DNA analysis proved with near certainty that the blood of eight-year-old Victoria ‘Tori' Stafford was found in Rafferty's 2003 Honda Civic, and mixed with his blood on a gym bag seized from the vehicle.

Rafferty has pleaded not guilty to first-degree murder, kidnapping and sexual assault in the April 8, 2009, disappearance of the Woodstock girl.