Autism, the prototypical autism spectrum disorder (ASD), is diagnosed in ∼15–20 per 10 000 people (4). The broader ASD category affects at least 60 in 10 000 children (5), but may be as high as 100 in 10 000 (6). Consistent with substantial heritability of ASD, risk to siblings of a proband with autism is 5–10%, substantially higher than population prevalence (7). A spectrum of severity is plausible due to the distribution of milder phenotypes in relatives of probands (8,9).
