Lily-Grace Davies was given little hope of a good life when she was born, but she has battled against the dark predictions and is knocking off the milestones.
The Auckland toddler was born with an extra - third - copy of chromosome number nine in many, but not all of her body's cells.
Arising from a cell-division error early in pregnancy, the disorder can range from mild or unnoticeable, to causing miscarriage or death soon after birth.
Only four Kiwi children have the extremely rare disorder - among about 160 known cases worldwide - and they will get together next weekend for a 4th birthday party.
Now aged 2 years and 11 months, Lily-Grace was born several weeks early, was light, at 1.7kg, and had malformed airways which required life-saving surgery at 4 weeks.
She has had five bouts of pneumonia requiring hospital admission, four procedures requiring general anaesthesia and she may need spinal surgery. Her joints, heart and eyesight are also affected and she suffers extreme anxiety, seizures and low immunity.
But mum Amanda Davies says Lily-Grace is progressing, against the odds.
"They said she wouldn't walk, talk, see or hear, she would have very little cognitive ability.
"She can walk, she can now spell 10 words, she can look at letters and read them. There are kids that go to kindy that can't do that. What I have proven is she can learn.
Only after having her tonsils and adenoids removed and grommets placed in her ears, at 18 months, did Lily-Grace start to respond to her parents' communication.
"She started making noises and copying mouth movements. And laughed. She had never been able to laugh as her larynx had been so floppy. In her first 18 months she would choke, vomit and not breathe as her throat would collapse from most movements.
"Her first word was 'Elmo' and then months with nothing else. Then came that beautiful call I longed to hear - 'Mummy'. 'Daddy' took her longer and instead she went for 'food', 'cat' and 'nana'."
But it was early speech-language therapy, singing and sign language that put Lily-Grace on the path to communication.
Now she loves to sing Twinkle Twinkle, and Old MacDonald.
"That's because of the music therapy we're paying for," says Davies, who adds that although they have had good support from Starship children's hospital, Lily's needs exceed what is provided by the state.
Q. What is trisomy 9 mosaicism?
A. An extremely rare disorder in which there are three copies of the ninth chromosome, instead of the usual two.
Q. How does it occur?
A. It results from errors during cell division early in the embryo stage of pregnancy
Q. Does it affect all cells?
A. No. Only some cells have the third copy and this can change over time, so different parts of the body can be affected.
Q. What are the effects?
A. There can be many and their severity varies. Common features include restricted growth during pregnancy, respiratory problems, mental disability, congenital heart abnormalities, muscle and bone deformities, and various organ problems.
Q. How rare is the condition?
A. Internationally, about 160 people are known to be alive with the disorder at present, but many more may have a very mild version and not know.
Q. Can it be treated?
A. The chromosomal abnormality can't be changed, but various operations, physiotherapy and other therapies are used to treat the effects.
Q. What is the outlook like for people with the disorder?
A. Some die in pregnancy, some in childhood and some have survived for decades.