There has been fresh criticism of forensic evidence at the Omagh trial.
A court artist's drawing of Sean Gerard Hoey
Dr Peter Gill, an exponent of the Low Copy Number DNA technique, conceded some of the results presented in the bomb trial were "valueless".
Mr Justice Weir warned Dr Gill about "blowing backwards and forwards" on "an important topic".
The judge said it was "very unhelpful" to give apparently contradictory evidence. Sean Hoey denies 58 charges, including 29 murders in Omagh in 1998.
Mr Hoey is a 37-year-old electrician from Molly Road, Jonesborough in County Armagh.
Low Copy Number DNA - a technique whereby DNA profiles can be obtained from samples containing only a few cells - is an important part of the prosecution case.
Dr Gill had been asked to comment on claims that control samples tested at the same time as parts of a device in Lisburn had come up positive for Mr Hoey's DNA type.
Twenty-nine people died in the Omagh bombing
That finding, said defence QC Orlando Pownall, should have meant that the tests were run again. The fact that they weren't meant the results were invalid, he claimed.
"I think it invalidates the result," Dr Gill agreed.
Dr Gill was also challenged over what appeared to be conflicting evidence on the reliability of Low Copy Number DNA testing.
Mr Pownall was questioning him about the amounts of DNA below which results could be relied on.
Giving evidence, Dr Gill said at a certain DNA level information taken from the results could be "informative".
But Mr Pownall pointed out that in papers Dr Gill had written on the subject he had said that at that level the results were "uninformative".
Mr Justice Weir intervened to say it "seems rather an important topic on which to be blowing backwards and forwards on.
'Shades of grey'
"One minute it's informative, the next it's uninformative." He asked which he should accept as expert evidence.
Dr Gill replied that it was a complex area in which there were "shades of grey".
The judge said: "When this evidence is presented on behalf of the prosecution no one talks about it in terms of shades of grey. It's put forward as evidence I can rely on."
This is not the first time the judge has intervened during the evidence of a forensic expert and on Friday, he once again commented that "this is not a scentific symposium, this is an important trial".
Mr Justice Weir then told Dr Gill that it was "very unhelpful for me to have you saying, 'informative one minute and 'uniformitative' the next", adding "why are you saying that?"
"I do not know," Dr Gill replied.
The case continues.