Experts are divided on whether a genetic mutation could prove child killer Kathleen Folbigg's innocence, an inquiry into her convictions has heard.
Genetic testing found Folbigg carries a novel genetic mutation known as CALM2 G114R, as did her two daughters.
Counsel assisting the inquiry, Sophie Callan, SC, said this "may cause cardiac arrhythmias and sudden death of young children".
Research supporting the theory that her children died of a cardiac condition, rather than being smothered by their mother, was previously endorsed by hundreds of expert scientists and doctors.
However, Ms Callan advised retired chief justice Tom Bathurst, QC, who is presiding over the inquiry, that "experts divide" on whether this mutation "could have a role in their deaths".
"No expert is expected to tell your honour that the CALM G114R variant definitely caused the deaths of either Sarah or Laura Folbigg," Ms Callan said.
"Equally, no expert is expected to tell your honour that the CALM2 variant could not possibly cause their death."
Folbigg was convicted in 2003 for the murders of three of her children and the manslaughter of a fourth over a 10-year period between 1989 and 1999 in the NSW Hunter Valley.
Caleb died aged 19 days, Patrick was eight months, Sarah 10 months and Laura 18 months.
The variant found in Folbigg and her two daughters was not detected in Caleb or Patrick Folbigg, the inquiry was told.
Craig Folbigg, the children's father, has continued to decline the invitation to provide DNA for testing, the inquiry heard.
Ms Callan said that while this genetic evidence was crucial, Mr Bathurst would also consider the mother's diaries and journals.
"Your honour is ultimately required to examine the whole of the evidence in considering whether there is a real possibility that one or more of the deaths of the children resulted from natural causes," Ms Callan said.
"It would be incorrect for the inquiry to only focus on the evidence going to the genetic variant."
The case against Folbigg had no physical evidence of smothering but largely hinged on diary entries she wrote at the time of the children's deaths.
Folbigg has always maintained her innocence and claims the children died from natural causes.
Two unsuccessful appeals and a judicial inquiry have so far failed to clear her name, but supporters hope today's inquiry will change this.
Childhood friend, Tracy Chapman, told the ABC that she spoke to Folbigg this morning and described her state of mind as "anxious".
"We're all hoping that the judicial system is capable of processing all the scientific and mental health-related information that's before them," Ms Chapman said.
"There's an overwhelming amount of positive evidence for Kathleen, so we're looking forward to having justice."
Folbigg listened to today's proceeding from the Clarence Valley Correctional Centre near Grafton.
Mr Bathurst will produce a report on whether there's any doubt surrounding Folbigg's convictions.
It is due next year, after a second round of witnesses will give evidence in February.
If there is doubt cast on her conviction, her case could be referred to the Court of Criminal Appeal.
The inquiry continues on Tuesday.
