A couple were told both of their daughters have a horrible life-limiting disease - but only one of them can be saved.
Ally Wallace and Jake Shaw found out Nala, two, and 11-month-old Teddi have a genetic condition affecting the 'white matter' of their brains.
The diagnosis also causes the loss of their children's physical and mental skills.
Nala, however, is unable to be treated for Metachromatic Leukodystrophy (MLD) as her brain has deteriorated too far already.
ChronicleLive reports she will continue to lose all functions and is not expected to live beyond childhood.
Teddi was tested for MLD after Nala was found to have the disease earlier this month.
As the 11-month-old's condition is not at an advanced stage, she is able to undergo life-saving gene therapy treatment in Manchester next month.
Ally and Jake, who live in Northumberland, have been are both carriers of the same 'faulty' genes and there was a one in four chance of them passing it on to their children.
The mum-of-two said: "We were absolutely devastated. I think it was even harder to hear that one of them can have treatment but one of them cannot be saved.
"It's just awful because Nala was such a character.
"She's gone from being this hilarious little girl, always singing along to Frozen every day, and now she can't even move. She can't feed herself or play with her toys anymore.
"Nala has saved Teddi's life. If Nala didn't have the disease, Teddi's disease wouldn't have been found quickly enough. We're going to carry on the legacy that she saved her sister's life.
"It's heartbreaking but it's the reality."
Ally first visited doctors with Nala after becoming concerned about her daughter's foot position and walking difficulties, which included walking on the inside of her foot.
Her mobility got worse and she developed a tremor in her hands. Doctors began looking at neuromuscular diseases and she was placed on a waiting list to see a paediatrician.
Ally, who works as bar trade, said: "Nala started to have these episodes. She would start crying and it was impossible to calm her down.
"She was shaking that much it was almost as if she was having a seizure. It happened two nights in a row and on the second night I decided to phone an ambulance.
"By this point she couldn't stand on her own and she could barely get herself off the sofa."
She said that Nala was taken to the Royal Victoria Infirmary (RVI) in Newcastle where she underwent an emergency MRI scan due to concerns that she may have had a brain tumour.
Ally and Jake, who works as a joiner, were then told that Nala had MLD.
"I didn't have a clue what it was. To start with I was relieved because for months I thought she had a brain tumour", said Ally.
"After the doctor left the room, Jake went onto Google and he went silent and white.
"He said 'I have just Googled it and it's not good, I have seen the life expectancy'. At that point I told him I didn't want to know."
As the condition was genetic, Teddi was brought into the hospital for tests and found to also have the disease.
Ally said: "It's incredibly unlucky that both parents have this gene. If we both have the gene then there is a one in four chance of passing it on and we have passed it on to both children.
"It's extremely rare, it affects one in 40,000. I have never heard of it before and I don't think anybody around me has heard of it."
According to the NHS, there was previously no treatment available for the brain disorder.
However, in February this year, the NHS struck a deal for the gene therapy, which reportedly the most expensive drug in the world.
Teddi will begin her treatment, which includes chemotherapy, at the Royal Manchester Children's Hospital on June 21.
Ally said: "Just the thought of her little body going through that doesn't bare thinking about. We're focusing on the fact this is going to hopefully save her life.
"They're going to remove one of her ovaries and freeze it for the future as the treatment she is having will make her infertile. It's just not something you think about doing when she's not even one. "
Nala's health has deteriorated over the last few months and she is likely to need a peg fitted, which will feed her, within the next few months.
"The life expectancy of child with MLD is between five to eight years.
Ally said: "I can tell she's frustrated because she can't understand why she can't do anything anymore.
She's only two so we can't explain to her what's going on either. She doesn't understand what's happening to her body and why she can't do anything anymore.
"She will be aware of everything that goes on around her but she won't be able to control her own body."
Ally has set up a page on GoFundMe to raise funds so they can take Nala to Disneyland and purchase equipment to make her more comfortable.
Some of the money will also be used to support Teddi as she goes through treatment.
She said: "We would like to take Nala to Disneyland if possible, we would like to do as much as we can with her before she goes."
Her close friend are also holding a fundraising event to raise money for the family.
Ally added: "I just want to say a huge thank you to absolutely everyone so far who has sent messages, donated and supported us."
