Video report by Helen Ford.
Sitting on the floor of a hospital consulting room, two-year-old Teddi Shaw appears oblivious to everything she is going through.
Teddi has travelled with her family from their home in Northumberland to Manchester, for a critical appointment to gauge the success of groundbreaking medical treatment.
Teddi is playing enthusiastically with toys: loading plastic bricks into a cart and drawing with coloured pencils; watched over by her specialist nurse.
The skills that Teddi is demonstrating will help to reveal her progress - exactly a year since she was discharged from hospital.
In 2022, Teddi became the first child in the UK to receive a pioneering gene therapy aimed, quite simply, at saving her life.
Teddi had been diagnosed with an inherited condition called metachromatic leukodystrophy, or MLD.
MLD is a progressive, life-limiting disorder where children cannot produce a crucial enzyme, leading to a build up of toxic material in the brain.
Over time, it leaves young patients unable to perform basic functions such as eating, seeing and talking.
Teddi's treatment, known as Libmeldy, involved removing her stem cells and replacing the faulty gene which causes MLD.
It was delivered by a team at Manchester University NHS Foundation Trust: the only centre in the UK to offer the therapy and one of a handful around the world.
Step forward to October 2023, and Consultant Paediatric Neurologist Dr Dipak Ram explained why Teddi's follow-up appointment is so important.
He said: "Teddi is coming up to two years and three months now and this is usually the time when MLD presents in children who have not been treated."
Dr Ram continued: "This appointment is really, really vital because this will tell us that the treatment is working and what we want to see is that she remains symptom free."
Assessing Teddi's progress through play is only part of the story.
During her visit to Manchester, blood samples are also taken, to measure the presence of the enzyme which will reveal the success of last year's treatment.
Those results, which come back several days later, are also promising.
More about Teddi's gene therapy:
The therapy Teddi received is known as Libmeldy
It replaces the faulty gene which causes MDL with functioning copies
Teddi was the first patient to receive the treatment on the NHS, outside of a clinical trial
At a cost of £2.8 million, Libmeldy was the world's most expensive drug' though the NHS negotiated what it called a 'significant confidential discount' to use it for health service patients.
While this is a particularly significant point in Teddi's medical journey, her monitoring will not end here.
Her development will be tracked by the team in Manchester for years to come.
Metabolic specialist nurse Rebekah Hutton has supported Teddi and her family from the start and told me what that relationship means to her.
She said: "To be part of Teddi's journey from the beginning of when she came to us, that first patient in the NHS to receive the treatment and to be part of that - and then still seeing her until she's eighteen years old, it just continues my passion and motivation to continue working with MLD."
While Teddi's future appears bright, her family must live with a heartbreaking reality.
Teddi's older sister Nala has the same, devastating condition and cannot be treated.
At the time of her MLD diagnosis, the disease in four-year-old Nala had progressed beyond the point where she was eligible to receive Libmeldy.
Since then, symptoms have become increasingly severe for Nala, who is now unable to swallow and is fed through a tube.
Their father Jake explained how he and Ally try to take each day as it comes, with the priority on spending time together as a family.
Mr Shaw said: "With Nala, we know the things she loves and Teddi loves the same things so we try and get Teddi involved and Nala with the stuff that she used to do."
He continued: "She's still here and we're not going to make her feel different just because she is different."
Call for testing
The girls' mother Ally is now adding her voice to wider calls for MLD to be included in the established UK-wide screening programme for new babies.
A few days after birth, parents of newborns are offered a blood 'spot test' which screens for a number of rare but severe conditions.
Mrs Shaw argues that since a treatment for MLD has become available, it should now be included on the blood test.
Teddi was able to be treated because the disorder had already been pinpointed in her older sister.
Mrs Shaw says routine screening would prevent this situation.
She said: "It shouldn't have to take for a child to be diagnosed, to them have a sibling that is able to get the treatment. It shouldn't take for one child to have to die, essentially, for younger siblings to live."
The decision on whether to include MLD on the blood spot test falls to an independent body called the UK National Screening Committee.
It has recently been reviewing the evidence on this issue and while it did not agree to include MLD in the screening programme at this stage, it concluded that further study is justified.
In a statement, the Department of Health and Social Care said: "The UK National Screening Committee does not currently recommend screening for this condition based on its latest review in June 2023.
"However a further review of the evidence will take place and the committee will report back in due course."
The conclusions of that further review are not expected until 2026 or 2027.
Want a quick and expert briefing on the biggest news stories? Listen to our latest podcasts to find out What You Need To Know...