Erin Cross, from Chester, received gene editing therapy in the USA after a Â£100,000 appeal raised the cash.
Doctors have told her parents she is now in remission after the therapy which took place in August.
Her mother Sarah Cross said: "It means she is now able to have a bone marrow transplant in Manchester."
She added: "I'm so glad I pushed for her to get on the trial here at Seattle, if I hadn't she wouldn't be here today."
Erin was treated for acute lymphoblastic leukaemia using CAR (Chimaeric Antigen Receptor) T-Cell therapy which re-engineers the cells in the lab to attack and kill cancer cells when injected back into the patient's body.
"We got a call from the hospital who told us the cancer cells have gone - we couldn't believe it as she has never come back clear from any treatment before," said Ms Cross.
"She is running round now like any six year old," she added.
Erin is due to return to the UK within the next month for a transplant at the Royal Manchester Children's Hospital where a donor has already been found.

Andrew Virco, 50, and Katharine Walker, 51, from Cambridge, died with two other Britons when the aircraft crashed into a glacier on South Island, on Saturday.
Ms Walker's daughter, Rebecca, 22, said the couple were on a "big adventure" that had been planned for some time.
"They do holidays but this was the big one... they had waited for," she said.
"New Zealand was her dream. Her and Andy had been planning it for a long time. It was what they would do for their 50th birthdays.
"It was their celebration together."
Ms Walker was head of radiotherapy at Addenbrooke's Hospital in Cambridge.
Her daughter, a university student, said: "She was everything. She had a special spin on life. She worked hard and she played hard. That's the way she lived her life.
"She affected so many people in so many good ways. The things she did at work were incredible.
"You just walked through town and everybody would be like 'hi, hi'. I couldn't go shopping without her seeing someone she knew. Cambridge is a big place but she affected so many people.
"Not having her here is just going to break so many people. It hasn't sunk in yet."
She described Mr Virco, an award-winning photographer, as "amazing".
"He took me into his world," she said. "He treated me like his daughter and you can't say much more than that. He's going to be missed."
Nigel and Helen Charlton, aged 66 and 70, from Hampshire, also died in the crash, along with Sovannmony Leang, 27, and Josephine Gibson, 29, both from New South Wales, Australia; and pilot Mitchell Gameren, 28, from Queenstown, New Zealand.
The recovery operation has been halted for the day after weather conditions at Fox Glacier deteriorated. Three bodies have been recovered.
The cause of the crash is being investigated.
The family of a six-year-old girl suffering from leukaemia are celebrating pioneering treatment which they say has "saved her life".
[NEXT_CONCEPT]
A couple who died in a helicopter crash in New Zealand were on a "dream" holiday to celebrate their 50th birthdays, their family has said.