Doctors at Great Ormond Street Hospital had said Charlie should be allowed to die in dignity and applied for permission to have his ventilator switched off.
Based on his existing knowledge of the case, Dr. Hirano told the court that he believed there was a "11% to 56% chance" that his therapy would improve the boy's condition.
The doctor will travel to the United Kingdom next week to examine Gard and present his evaluation of Gard's prospects for improvement with treatment to the court.
British doctors determination to end Charlie's life, even though hospitals in both the USA and the Vatican have offered to take him and his parents have raised over £1.3 million in public donations to handle the necessary logistics themselves, has proved controversial around the world.
Speaking on behalf of "the entire family and their supporters", spokesman Alasdair Seton-Marsden said their attention had been drawn to "certain threats" against the judiciary, barristers representing Great Ormond Street, and against doctors and nurses at the hospital.
Image Charlie has a rare genetic condition
Charlie's parents have described the "Save Charlie Gard" petition as evidence they should be given the right to make the final decision over their son's treatment.
An American doctor will head to the United Kingdom next week to examine Charlie Gard, the 11-month old boy suffering from a rare genetic disorder.
Charlie suffers from a rare terminal illness, mitochondrial DNA depletion condition, but his parents Chris Gard and Connie Yates believe the new experimental treatment developed by Dr Hirano could save his life.
Pro-life pastor Rev Patrick Mahoney has flown to Britain prayed with the parents at Charlie's bedside in Great Ormond Street and is attending the High Court hearings in support of the parents.
"Where there's life, there's hope, and we will continue praying for Charlie and his parents", Pavone said.More news: You can get a dozen Krispy Kreme doughnuts Friday for 80 cents
They boy's parents are fighting an emotional legal battle with Britain's most famous children's hospital over whether trying an experimental treatment is in Charlie's best interest.
Amid questioning from Judge Nicholas Francis, Charlie's mother, Connie Yates, accused Francis of misquoting her earlier statements about Charlie's quality of life. On Thursday, they walked out of a courtroom after arguing with the judge, but returned later in the day.
Francis offered a reassuring word, saying: "I understand you walking out because it is a desperate situation". Chris Gard punched a table, while his wife said: "We said he's not in suffering and in pain".
(AP Photo/Matt Dunham). The parents of critically ill baby Charlie Gard, Connie Yates and Chris Gard, right, arrive at the High Court in London, Friday, July 14, 2017.
In a reflection of the tension surrounding the case, Chris Gard and Connie Yates stormed out mid-hearing when they disagreed with remarks by the judge.More news: Roku to File for IPO This Year
Detail of the plan emerged on Friday during the latest round of a legal fight between Great Ormond Street bosses and Charlie's parents.
The parents of a baby with a rare disease are returning to a court in London, hoping for a fresh analysis of their wish to take the critically ill child to the United States for medical treatment.
Charlie Gard suffers from a rare genetic disease which has left him brain-damaged.
His request for the measurement came after Judge Francis sparked outrage from Charlie's parents when he said that they had previously said it's okay if Charlie dies.
"GOSH seem to want to exclude the parents at every stage", he added.More news: Chiefs will likely face Ezekiel Elliot despite rumored suspension
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