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.
Judges have heard that Charlie, who was born on 4 August 2016, has a form of mitochondrial disease, a condition that causes progressive muscle weakness and brain damage.
Columbia University medical school specialist Michio Hurano on Monday examined Charlie Gard with British colleagues ahead of a High Court hearing on new experimental treatment and a possible move to the United States for the terminally ill 11-month-old British boy, whose Great Ormond Street doctors are poised to pull the plug after earlier British and European court rulings.
Dr Hirano has said he believes there is now a better chance the treatment would produce a meaningful improvement than there was when he gave evidence three months earlier.
Hirano is a professor of neurology at Columbia University Medical Center and specializes in rare genetic conditions.
The new High Court hearing was requested after the Vatican's children's hospital and the U.S. hospital said a new experimental protocol might work for Charlie, whose parents have been trying to secure a move to the US.
Hirano is one of the pioneers of nucleoside - an experimental therapy which is being sought by Charlie's parents, Chris Gard and Connie Yates. After much debate in court on Friday over whether Charlie's parents will be present for this, the Judge announced that Yates will be allowed to attend.
Specialists at Great Ormond Street say that the therapy is experimental and will not help Charlie.
The emotional case went to the UK High Court last week after the hospital requested a new hearing to consider "new evidence relating to potential treatment for his condition".
It has prompted a fierce debate around the world about medical ethics and whether the hospital treating the child or his parents should determine his fate.
After a series of hearings and appeals in several courts, the European Court of Human Rights decided on June 30 not to intervene in the case, which upheld a British Supreme Court decision that the hospital could discontinue life support to Charlie and he could not be transferred.
He is due to travel to London on Tuesday to examine Charlie.