A faulty gene could be responsible for some cases of cot death, or sudden infant death syndrome, according to a group of US researchers.
Tests on Amish families from Pennsylvania have found a new disorder that causes sudden death and sometimes malformation of the genitals.
The findings, in this week's Proceedings of the National Academy of Sciences, could explain some of the thousands of unexpected deaths each year.
The researchers have named their newly found disorder sudden infant death and dysgenesis of the testes syndrome (SIDDT). They say babies must inherit two flawed copies of the gene to develop the symptoms, including organ dysfunction.
Such babies die before they are a year old from sudden cardiac and respiratory failure.
Dietrich Stephan of the Translational Genomics Research Institute in Phoenix, Arizona, and colleagues at the Clinic for Special Children in Strasburg, Pennsylvania, found 21 infants from one Amish community died from SIDDT in two generations.
To find the gene, they analysed DNA from four affected infants, their parents, siblings and other relatives.
They found that males with the syndrome may have underdeveloped testes, but girls appear normal, with normal female hormones. But both male and female infants with SIDDT die suddenly at the same age.
"One infant died in the hospital while awake and attached to a cardiac-respiratory monitor," the researchers said.
The monitor showed the child's heart and breathing stopped at the same moment.
This is one of the first genetic sub-classifications of Sids, said Dr Stephan: "It's going to be helpful in offering parents answers for sudden infant deaths, recognising predisposition early, and hopefully saving a number of these babies."
Although Sids is used as a general term for unexplained infant deaths, doctors have long suspected a number of different factors.
Campaigns in Britain and the US to educate parents and to put babies to sleep on their backs instead of their tummies halved such deaths, but did not eradicate them. In Britain in 2002, 342 babies died in unexplained circumstances.
