Key points from article :
Mila Makovec, now aged eight, was diagnosed with fatal and untreatable Batten disease.
Doctors from Boston Children's Hospital in the US created the tailor-made drug for Mila.
Mila was treated with the drug - named Milasen - on 31 January 2018.
Drug sticks to the forming mRNA and stops it being cut in the wrong the place.
Details have been published in the New England Journal of Medicine.