Expert testimony has been hailed as a potential breakthrough in the appeal by Amanda Knox and Raffaele Sollecito against the murder of Meredith Kercher.
They are serving 26 and 25 years respectively for murdering British student Meredith in the university city of Perugia in 2007.
But independent experts in Italy have told the court that crucial forensic evidence at the trial was unreliable.
They could not rule out that DNA traces on a knife and clothing which formed the basis of the prosecution case were from contamination.
They also questioned whether proper procedure had been followed.
21-year-old student Meredith was found with her throat cut in the house that she shared with Knox in Perugia.
The judges said it was the result of a sex game that had got out of hand.
Both Knox and Sollecito deny they were responsible.