Sally Clark lost both her infant sons shortly after their births. In 1996, 11-week-old Christopher was put to bed and never woke up. Two years later, 8-week-old Harry was found dead in his bouncy chair.

Doctors initially concluded that Christopher had died of sudden infant death syndrome (SIDS) — in which a seemingly healthy baby dies without warning or an obvious cause. But after Clark’s second child died, prosecutors in the United Kingdom put her on trial for allegation of murder.

An expert witness for the prosecution claimed the chance of two cases of SIDS, in an affluent family like hers, was astronomically high — 1 in 73 million. But her defenders said that the numbers assumed that SIDS strikes at random.

A study published in the Lancet shows a link between SIDS and a rare genetic mutation that would make some families more vulnerable than others — providing a possible explanation for situations like Clark’s.

The research involved 278 infants who died of SIDS and 729 healthy controls. Four of those who died of SIDS had a variant of a gene called SCN4A associated with an impairment of breathing muscles, while no babies in the control group had it. Authors Michael Hanna from the United Kingdom’s Medical Research Council’s Center for Neuromuscular Diseases and Michael Ackerman of the Mayo Clinic wrote that these mutations are extremely rare and typically found in fewer than 5 out of 100,000 people.

The paper stressed that genetics is just one of the factors that could contribute to SIDS and explained the “triple risk hypothesis” of the condition where a vulnerable infant, during a critical period in development is exposed to an external stress.

The researchers suggest that the genetic mutation could leave some babies with weaker respiratory muscles so that they would be unable to correct their breathing as quickly or as well while they are sleeping in reaction to things like tobacco smoke or a stressful sleep position.

“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,” Hanna said.

They noted that the study has several limitations, including the fact that all the participants were white and of European ancestry.

Since 1994, the National Institute of Child Health and Human Development has been advocating that parents put their babies to sleep on their backs to reduce the risk of SIDS.

As for Clark, her conviction was overturned after she had served three years in prison. She died in 2007 at her home.