Cot death may be linked to rare genetic mutations associated with impaired breathing, research has shown.
British and US scientists have found the first clear evidence that genetics plays a role in sudden infant death syndrome (Sids).
The study focused on mutations in the SCN4A gene which helps regulate the muscular control of breathing.
Defects in the gene are associated with a range of neuromuscular disorders, some potentially life-threatening, that make breathing or speaking difficult.
Typically SCN4A mutations are very rare, occurring in fewer than five people in every 100,000. But the study uncovered harmful mutant versions of the gene in four of 278 cot death victims.
Having one of the gene variants may leave some children with weaker breathing muscles, said the researchers.
Faced with extra stresses to breathing such as tobacco smoke, getting entangled in bedding, or a minor illness or airway obstruction, they may be less able to correct their breathing, cough or catch their breath.
Professor Michael Hanna, from the Medical Research Council Centre for Neuromuscular Diseases at University College London, said: "Our study is the first to link a genetic cause of weaker breathing muscles with sudden infant death syndrome, and suggests that genes controlling breathing muscle function could be important in this condition. However, more research will be needed to confirm and fully understand this link.
The scientists stressed that faulty genes are probably not the sole cause of cot death, and safe sleeping measures were still essential to ensure a baby's safety.
Sids, defined as the unexpected death of an apparently healthy child, can be heart-breaking and traumatic for parents.
An estimated 300 cot deaths occur each year in the UK, and 2,400 in the US. Typically, the syndrome strikes infants aged two to four months.
Although the cause of cot death is unknown, it is widely believed to be linked to breathing problems. Putting babies to sleep on their back, and not having them sleep in the same bed as a parent are both measures known to reduce the risk.
For the study, the researchers looked at tissue samples from 278 British and American children whose deaths remained unexplained despite thorough post-mortem investigations.
DNA from the samples was compared with that from 729 healthy adults with no history of heart, lung, or neurological disease.
The findings are reported in The Lancet medical journal.
Not all SCN4A mutations are potentially lethal, said the scientists. The dangerous ones affected an "ion channel" protein that regulates the flow of electrically charged sodium into skeletal muscle.
In a linked comment accompanying the research paper, Dr Stephen Cannon, from the University of California at Los Angeles, said: "A better understanding of the causes of Sids is needed to identify infants at high risk and to develop interventions and guidelines that will prevent Sids for all infants... Overall, the evidence is compelling that variants of SCN4A with disruption of channel function are over-represented in Sids."