Animal rights activist Mel Broughton is standing trial in Oxford
The defence in the trial of an animal rights campaigner accused of fire-bombing Oxford University buildings has questioned the use of DNA evidence.
Mel Broughton, 48, from Northampton, is on trial at Oxford Crown Court and denies all the charges against him.
A key defence witness has challenged the validity of the use of microscopic DNA samples found on a petrol bomb at Templeton College in February 2007.
Doctor Allan Jamieson told the court such minute samples were "unreliable".
Earlier in the trial the prosecution said the so called "low-template" or "low count network" (LCN) DNA samples found on the petrol bomb at Templeton College matched Mr Broughton's DNA profile.
The LCN DNA evidence is crucial to the prosecution's case.
On Friday the jury spent the day listening to evidence from a number of forensic experts on this type of DNA analysis.
They heard that last year the use of LCN DNA evidence was suspended while a report was carried out into how it was independently assessed.
Dr Adrian Linacre who co-wrote the report, which was subsequently published in April 2008, appeared as an expert witness for the prosecution.
He explained to the court how LCN DNA analysis worked. He also said the use of such complex statistics can cause "problems for jurors".
Dr Jamieson, who was called as an expert at the Omagh bomb trial in 2007, was called as a witness for the defence.
The DNA evidence being disputed was taken from a sample found on bombs found at Templeton College
He said there were "insurmountable" problems with low template DNA analysis.
With regards to the microscopic sample which was found to match Mr Broughton's DNA, he said: "This sample of DNA is the minimum size that can be measured."
He disputed prosecution claims that there was only a one-in-one billion chance that the DNA found could belong to someone else.
"It was possible that the sample could match Mel Broughton," he said.
"But it was also possible the DNA belongs to someone other than Mr Broughton."
He also said he was not certain it was sufficient evidence against a defendant.
Mr Broughton, of Semilong Road, Northampton, denies charges of conspiracy to commit arson.
He has also pleaded not guilty to an alternative charge of possessing articles with intent to destroy property and keeping an explosive substance with intent.
The case continues on Monday.