Ōrewa 9-year-old Corin Copeland has a heart condition that has been reported in only 40 other people in the world - about one out of every 200 million human beings.
The condition - a lack of elastin in his arteries which causes them to close up - is so rare that it doesn't even have an agreed name.
But it is known that it can be genetic. Corin's mother Myka Copeland, 36, has been found to be a carrier of the gene, and Corin's older brother Tyler had the same condition until he died in 2007 aged just 11 weeks.
The family is speaking out about the condition because Corin's father Jon Copeland, a project manager with Landmark Homes, is working with Landmark's North Shore and Rodney franchisees Debbie and Paul Brett to build a house that will be auctioned to raise money for the child health research charity Cure Kids.
Corin, who attends Ōrewa North Primary School, lives a pretty normal life but has to make sure he doesn't push his heart too hard.
He had open-heart surgery when he was two and a half weeks old and again when he was three months old. Since then, small tubes called stents have kept his main aortic artery open.
He has been able to do lifesaving and play rugby, but was told late last year that he had to give away the rugby because it put too much strain on his arteries.
"We're figuring out another sport at the moment," Jon Copeland said.
He had a teacher aide at school until last year "to make sure he didn't overdo it", but was now old enough to manage by himself.
"The teachers look out for him, and when we do school trips and things like that we have to be part of it," his dad said.
His mother Myka said no one else in her wider family was known to have carried the gene and her father had died without being tested.
Corin's older sister Kamdyn, aged 11, does not have the gene.
Starship Hospital cardiologist Dr Tim Hornung said "a handful" of other people in New Zealand were known to have the condition, so the figure of 40 reported cases was likely to be an under-estimate of the actual numbers with the condition worldwide.
"Some people call it inherited elastin arteriopathy," he said. "That's really just describing what we see. It's inherited sometimes, and arteriopathy means it's a disease of the arteries.
He said the condition was variable.
"We have seen some children where the disease seems to be very aggressive early in life, and then as they have got older it's become less aggressive," he said.
"With rare things like this it's quite hard to say what is the usual pattern."
The four-bedroom, 236-square-metre Landmark house that will raise funds for Cure Kids is being built at 37 Kano Way, Scotts Landing, at Hobsonville Point.
The house will be auctioned on September 27.