By Kevin Bishop
BBC News, New York
A boy from Barnet in North London is undergoing breakthrough treatment in a US hospital for a rare form of cancer.
Jack is in the middle of his cancer treatment
It is the coldest March day for years in Manhattan, and the Brown family are battling the bitter wind to walk the four blocks from their temporary home, the Ronald McDonald House, for their daily visit to Sloan-Kettering Cancer Centre on the banks of the East River.
Bundled up against the cold, Jack Brown looks like any other five-year-old.
It's only when he reaches the warmth of the hospital that the extent of the treatment he has been undergoing is revealed.
Jack has lost his hair; a scar snakes down towards his forehead and a surgical dressing hides a tube that is used to feed the doses of radioactive antibodies that he is being fed to fight and kill the cancer that is attacking his brain.
But the first thing you notice is not that. He is shy and a little tired this morning, but peeping out from behind his mother's legs, the cheeky five-year-old's smile shines through.
Yvonne and Richard Brown, both police officers in London, first noticed symptoms of Jack's condition two years ago.
He has neuroblastoma, a solid tumour cancer that began in the nerve tissue of the abdomen and rapidly spread through his body, making it difficult to treat.
His reaction to the initial treatment was good, but in November he had a major relapse and was told in London that there was no treatment available at this stage of his illness in the UK.
His parents knew that an experimental treatment was available in the US and, fearing that he would soon be too ill to travel, headed straight for Heathrow airport.
"In the UK we knew that there was nothing but palliative treatment available", Richard Brown said.
"But here in New York there was still the chance of a cure in that the hospital had dealt with 10 children over the last three years using a new technique and that technique had kept all 10 children alive."
The treatment, which involves a large dose of radioactive antibodies directly into the brain and spinal cord, offers children a chance once regular chemotherapy methods have been exhausted.
But it is experimental and therefore carries high risks.
However Yvonne, who has followed every stage of development in Jack's illness, sees this treatment here in the US as the last chance for him, and many others like him.
"The body could not stand the same amount of chemotherapy over again, and therefore they've had the treatment and they're not strong enough to undergo it again.
The family are raising money through a special website
"An adult could not undergo this treatment at all.
"It's only because children are so resilient that their bodies can cope with it."
So, for three months now the family, with Jack's brother and sister Connor and Rhian, have been living in one room in a home, provided and paid for by McDonalds, that specialises in helping the families of children attending Sloan-Kettering for similar conditions.
Sitting in their room, watching the snow flurries outside, Jack and his brother tussle over a football.
Richard and Yvonne have found schools for all three children here while Jack's treatment continues.
The room is small for five people, but it's a haven for the family.
From here they have been running a campaign to raise awareness for the cure, and money to fund the treatment.
Looking out through the huge windows that look out over New York's skyscrapers, they appreciate all the attention people have been giving their cause.
"Knowing that people have an affinity with Jack without actually knowing him but perhaps knowing how special he is, or how special he is to us has kept us going"
Sitting on the bed, Yvonne writes her blog, charting Jack's treatment and the progress of the appeal.
They need to raise another $750,000 to complete the treatment.
Yvonne clicks on a link to a song by Phil Ryan, written for the appeal as a message from Jack to his parents. Jack sings along quietly to himself as his mum gently kisses his head.