The retrial of Amanda Knox and Raffaele Sollecito over the murder of Meredith Kercher has begun - but neither are there.

The US student and her former boyfriend were freed on appeal after their convictions were overturned in 2011.

But Italy's supreme court overturned the acquittal in March, suggesting that that Rudy Guede, a drifter also convicted for the murder, did not act alone, and describing "shortcomings, contradictions and inconsistencies," in the appeal court's verdict.

The court in Florence is expected to re-examine forensic evidence with the trial judge in a position to order new DNA tests, if that's deemed neccessary.

Lawyers said the court may order tests on a tiny biological trace on a knife suspected of being used in the killing which was overlooked during earlier trials.

A new appeal verdict will now be reached, possibly by Christmas, before the case returns to the supreme court. If Knox is found guilty and the supreme court upholds the verdict, she may face an extradition request, although the US would likely refuse to hand her over.

Nick Squires, Rome correspondent for The Telegraph newspaper, spoke to Newstalk's Breakfast:

Francesco Maresca, a lawyer representing the Kercher family, said it was possible the new presiding judge would order a DNA test on a tiny biological trace found on the alleged murder knife alongside other traces, which was only discovered by appeal court-appointed experts.

"It was considered too small to test at the time, but there are new kits now," he said. "Let's see how well it was conserved."

Mr Maresca said the experts appointed by the appeal court who questioned DNA evidence found on the knife and on Kercher's bra clasp, had been rightly challenged by the Supreme Court.

"They were unprepared for something that important and probably influenced by the defence," he said.

Carlo Dalla Vedova, a lawyer for Knox, defended the experts' work and said he would ask that they be summoned to defend their work at the new trial.

"The supreme court's criticisms of the acquittal are all wrong," he said.

Giulia Bongiorno, a lawyer representing Sollecito, said she would request an examination of a stain found on Ms Kercher's pillow, suspected to be sperm, which was never tested.

After giving a stream of interviews in recent weeks in which she has proclaimed her innocence, Knox, now 26, has said she will not travel from her home in Seattle for the trial, while Ms Bongiorno said Sollecito would attend later hearings.

"Knox's justification of her absence suggests she considers herself a victim of Italian justice, which is intolerable," said Mr Maresca.

Knox has said she would like to visit Ms Kercher's grave, but Ms Kercher's family said last week she should stay away Meredith's last resting place.

Mr Maresca said Ms Kercher's sister Stephanie had planned to attend the hearing, but had decided to stay with her parents in Coulsdon, Surrey, because "they need her support".

Meredith (21) was killed six years ago in the university town of Perugia.