Identical twins can be genetically different, which could explain why they do not always share disorders such as schizophrenia or autism, according to a study published in March in PLoS One.

Small duplications or deletions of DNA regions — called micro-copy number variations — may not lead directly to disease, but could raise the risk of autism when combined with other mutations, according to a study published in March in the American Journal of Medical Genetics Part B: Neuropsychiatric Genetics.

Rare mutations with strong effects play a key role in autism and schizophrenia, according to a study published in February in PLoS Genetics. The study identifies rare harmful mutations in three candidate genes that are more common in individuals with one of the disorders than in controls.

Spontaneous mutations that change a single DNA base account for a large proportion of cases of unexplained mental retardation, according to a study published in the December Nature Genetics.

Three themes — heterogeneity, phenotype, and trajectory — are beginning to emerge in diverse lines of inquiry into autism, according to speakers at a satellite symposium of the Society for Neuroscience annual meeting in San Diego.

The Simons Simplex Collection (SSC), a database of genetic and clinical information from families that include one child with autism, has gathered data from more than 2,000 different families, researchers report in the October issue of Neuron.

Spontaneous harmful mutations are more frequent in individuals with autism and schizophrenia, according to two studies published in September.

Changes in diagnostic practices, more active neighborhood networks, and an increase in the number of older parents may all contribute to the massive rates of autism in California, says a group of social scientists. But the numbers still don’t add up.

People with autism harbor more copy number variants (CNVs) — deletions or duplications of large chunks of DNA — compared with controls, but only in the protein-coding regions of the genome, researchers reported Wednesday in Nature. The study also pinpointed more than 100 new risk genes for autism.

The older a grandmother was when she bore her grandchild’s mother, the greater the child’s risk of autism, according to a study published last month in PLoS One.