Scientists have discovered the cause of a fatal form of brittle bone disease.
Brittle bone disease can vary in its severity
All forms of the condition, also called Osteogenesis Imperfecta, weaken bones causing frequent fractures.
But US National Institutes of Health researchers found a genetic defect in a protein which helps produce collagen causes a fatal form of the disease.
The researchers say the findings, published in the New England Journal of Medicine, will allow affected families to be tested for the gene.
Brittle bone disease occurs in around one in 15,000-20,000 births.
In the most severe forms, which affect around a quarter of cases, children may die at or shortly after birth.
People with other forms of the disease can lead a relatively normal life, but have bones that fracture more easily.
The NIH team analysed a gene which contains the information for cartilage associated protein, or CRTAP.
The role of CRTAP is not fully understood, but it is known to be part of a complex of proteins involved in the chemical transformation of collagen from simple protein "chains" into its final form.
The NIH researchers carried out tests on mice and discovered mutations in the CRTAP gene accounted for a recessive form of the disorder - which means two copies of the affected gene are needed to cause disease.
The team, led by Dr Joan Marini, then looked at tissue samples from three babies with the condition who had all died before they were one, but where doctors had not known the cause of their disease.
One sample had been stored for almost 15 years, in the hope that a cause would eventually be discovered.
Tests showed that, in each case, the CRTAP genes were not working and the babies completely lacked the CRTAP protein.
In both of the two sets of parents who were tested, each parent carried one mutant CRTAP gene and one normal CRTAP gene.
The researchers suggest this form of the disease may affect between 2-3% of people with brittle bone disorders.
While there is no treatment, the findings could allow doctors to test families who have already lost a child to brittle bone disease to see if they are carrying the recessive CRTAP gene.
Duane Alexander, director of the National Institute of Child Health and Human Development, said: "This discovery provides a basis for counselling families that have lost a child to this previously unexplained form of Osteogenesis Imperfecta.
"It also offers insight into a crucial step needed in the formation of bone and other tissues."