Doctors have asked the High Court to hear new evidence in the case of terminally ill Charlie Gard.
The 11-month-old is on life support at Great Ormond Street Hospital (GOSH).
Charlie has an exceptionally rare genetic condition called encephalomyopathic mitochondrial DNA depletion syndrome (MDDS).
Although he appeared perfectly healthy when he was born, his health soon began to deteriorate.
His condition also means he is unable to breathe unaided, which is why he needs to be on a ventilator.
Charlie's heart, liver and kidneys are also affected, and his doctors say it is not clear if he feels pain.
Charlie's parents, Connie Yates and Chris Gard, from Bedfont in west London, want Charlie to have an experimental treatment called nucleoside therapy.
A hospital in the US has agreed to offer Charlie the treatment, and Charlie's parents have raised funds to take him there.
But Charlie's doctors at GOSH do not think this is the right care for Charlie.
They say they have explored various treatment options, including nucleoside therapy, and none would improve Charlie's quality of life.
They say Charlie's life support should be switched off and he should be allowed to die.
Charlie's doctors say his brain is extensively damaged at a cellular level.
They say the US clinician offering the experimental treatment agrees it will not reverse this brain damage.
Charlie's parents then appealed against the decision, but courts ruled that the original decision should stand and that it would be in Charlie's best interests to be allowed to die with dignity.
GOSH says it has applied to the High Court for a fresh hearing "in light of claims of new evidence" relating to potential treatment for Charlie's condition.
Two international hospitals and their researchers have been in touch with GOSH with more information about nucleoside therapy.
Meanwhile, the Pope and US President Donald Trump have offered to intervene and support Charlie's parents' plea.
The president of the Royal College of Paediatrics and Child Health, however, says such interventions from high-profile figures, no matter how well-intended, are "unhelpful".
Charlie's doctors say their view has not changed - they still think the experimental treatment would be unjustified.
The therapy is a treatment, not a cure. And it is highly experimental.
It has been used on some patients, but none with Charlie's type of MDDS.
There haven't yet been any trials in mice to see if it would work in Charlie's condition, which is caused by a mutation of a gene called RRM2B.
Doctors say it doesn't look as though there will be time to run such tests before reaching a decision about Charlie's care.
GOSH did apply for ethical permission to attempt nucleoside therapy on Charlie.
By the time that decision was made, however, Charlie's condition had greatly worsened and the view was that his brain damage was too severe and irreversible for the treatment to help.
Prof Julian Savulescu, an independent ethics expert at the University of Oxford, said: "More than six months have passed since experimental therapy was first considered. We don't know how bad Charlie's brain damage is now. Whether experimental therapy is still warranted depends on whether there remains any prospect of any meaningful life."
The High Court is now being asked to judge the evidence on this crucial point.