The girl, who suffers from Hurler Disease which is caused by a faulty gene, will have her own bone marrow transplanted back into her body after it has been genetically altered in a laboratory.
Last week her bone marrow was taken out and transported to a laboratory at the Paterson Institute at the Christie Hospital in Manchester, to be treated.
A normal gene was inserted into it, which doctors hope will override the faulty one, and the marrow will be returned to her in the operation later today.
The operation, which will take place at the Royal Manchester Children's Hospital, could help hundreds of people if it is successful.
Professor Mike Dexter, Director of the Paterson Institute, who is part of the team carrying out the operation, said it was a medical breakthrough.
"It is the first gene therapy operation on this group of patients in the world," he said.
"There are very few people suffering from the disease in the UK, and they usually die before they are 10 years old.
"We don't know if the operation will work, but if it does we will be very pleased.
"We should know within a fortnight if it has been a success," he added.
The girl's mother, who has asked to remain anonymous, said: "We are very relieved that she has been given the chance to have this treatment because there was no other option for her and this is her only chance."