South Tyneside siblings Callum and Lana Meek may each be one in a billion.
For they are almost alone in having an ultra-rare genetic condition.
Callum, nine, and Lana, three, of Gainsborough Avenue, Whiteleas, have a CSTB gene fault - a condition which affects their ability to see, move and communicate.
They are thought to be the only sufferers in Britain – and few other cases are known globally.
With no known cure and limited research, the affliction is classed as life-shortening.
By chance, their brothers Ethan, 18, Luke, 13, and Drew, two – the only other sibling to be tested – are thought to be free of the genetic defect.
Their parents Jemma, 33, and Aaron Meek, 40, a self-employed tiler, say they accept a cure is unlikely to be found in time to help Callum and Lana.
But their devotion to them has received a recent boost when they won the support of experts at St Oswald’s Hospice in Gosforth, Newcastle.
They are now able to enjoy a three-day break each month while Lana and Callum are looked after by staff.
Jemma, the children’s full-time carer, said: “The condition is too unknown to ever really start looking for a cure, I wouldn’t know where to look.
“We see doctors at hospitals in Newcastle, mainly the RVI, every few months, but it’s only to ensure they are doing OK and not because of any better treatment they can be given.
“Callum and Lana can’t communicate at all, they are non-verbal and unable to show us what they want in any way at all.
“We have learnt to know when they are uncomfortable or if they are not feeling well.
“It is a difficult process to care for them but it is something that is very worthwhile.”
The children were born with the condition but Callum was seven years old before he was diagnosed. By then doctors had carried out numerous tests for common conditions such as cerebral palsy, but all proved negative.
The breakthrough started when a mouth swab taken when he was four helped to map his genetic make-up.
But it was another three years before laboratory tests revealed the truth.
Aaron and Jemma were then tested and each was found to have a CSTB gene of which one half was faulty – an almost unique coincidence.
Callum and Lana then inherited each parent’s faulty half gene, creating a whole defect gene that caused their condition.
Once Callum was correctly diagnosed, a test for Lana confirmed the condition in just two weeks.
The CSTB gene provides instructions for making the cystatin B protein, which reduces the activity of a particular enzyme.
Its specific function is unclear, but it may help protect the cells’ proteins from an enzyme that leaks.
Of St Oswald’s, Jemma added: “It is the only place that can cater for their complex needs.
“Callum previously attended a respite centre but the care was led by carers and wasn’t fit for his medical needs.
“At St Oswald’s the service is nurse-led, so I know that I am leaving the children in safe hands.
“Often, people are too scared to look after Callum and Lana as they have such complex needs.
“However, straight away the nurses and other staff at St Oswald’s took over the children’s care.
“The care is great and it made leaving them overnight so much easier knowing that they settled there so easily.
“As well as giving my husband Aaron and I a break, the children enjoy going to the hospice.”
A spokeswoman for St Oswald’s said: “We provide short breaks and respite for children with incurable conditions from across the region, plus support for families.
“It’s the only service caring for children and young adults from South Tyneside that provides the medical support and respite that is needed for Callum and Lana.
“They started using this service earlier this year, and now come to us for three nights each month.”
At the hospice, Lana takes part in music therapy, and Callum enjoys short trips.
St Oswald’s provides care to adults, young adults and children from across the North East.
No charge is made for the service, ensuring care is available for all, and the £7million it needs annually to support its services is raised through voluntary donations.