A baby boy from West Mersea in Essex gave his big sister the ultimate gift - life. Jacob Davis was just 10 months old when he donated his bone marrow to his sister Seren-Rose.
She was diagnosed with MPS last December - it's a rare, debilitating disease that means her body fails to produce a certain enzyme which delays her development and causes her to slowly deteriorate. It affects one in every 25,000 children.
The bone marrow transplant in May was a success and Seren-Rose returned home after eight weeks in hospital.
The transplant won't cure the disease or reverse the damage it's already caused but it will stop it progressing any further. Her parents say they are seeing Seren-Rose improve everyday.
Her parents Simon and Elouise said: "With Seren we knew without the transplant she would die at a young age. There was a risk of one in 10 that she wouldn't survive.
"We had to give her the best shot of a future and we were assured that it was a safe procedure for Jacob. He was back the same day, but it was not as smooth for her - but definitely a risk worth taking.
"I want him to know about it and be proud of it when he's older. He's done an amazing thing, he's saved his sister's life. It's a sibling bond that's even stronger than the norm and we are just hugely proud of both of them. And now he will grow up with his sister instead of us just telling him what she was like, so it's really good.
"Without the bone marrow transplant she wouldn't be improving, she'd just be deteriorating even worse - so she is starting to get better which is great."