WEDNESDAY, Feb. 4 (HealthDay News) -- Children who have the most common form of mental retardation -- called nonsyndromic mental retardation -- appear to have a genetic mutation that may contribute to the condition, new research finds.

What's unusual about the mutation is that it occurs during development of the child and is not passed on by the child's parents, the researchers said.

"Nonsyndromic mental retardation is a very common problem -- it involves about 3 percent of the population," said lead researcher Dr. Jacques L. Michaud, of the Center of Excellence in Neuromics at the University of Montreal in Canada. "It is the most common mental handicap in children."

Children and adults who have nonsyndromic mental retardation have no physical abnormality, they look like any other child or adult, but they nonetheless have the condition, Michaud said.

"We have good reason to think genes are the cause of this syndrome," he said.

The findings were published in the Feb. 5 issue of the New England Journal of Medicine.

For the study, Michaud and his colleagues zeroed in on the SYNGAP1 gene -- which produces a protein critical for learning and memory -- in 94 patients with nonsyndromic mental retardation. The researchers identified mutations in the gene in three children.

These are new mutations, Michaud said. "Mutations you find in the kids, but you don't find in the parents," he said. "The mutation arises in the development of the kid."

The mutations involve the brain, especially those areas that affect the development, function and connection between neurons, Michaud said.