Heterogeneity and homogeneity across the autism spectrum: the role of development.

Autism is a lifelong, highly prevalent, and strongly genetic disorder defined by impairments in social and communicative function and by pronounced behavioral rigidities. 1 Despite common areas of impairment that define autism as a condition, individuals with autism show a vast clinical variability in the expression and severity of their symptoms. This heterogeneity spans the entire range of IQ and language function and a wide array of communicative, social, and behavioral disabilities. At the level of genetics, the variability seems comparable: although more and more susceptibility loci are being identified, each is thought to account for only a small number of overall cases (e.g., see reference 2 ). Likewise, de novo mutations may play a causal role in a relatively large percentage (E10%) of individuals with autism who do not have an affected first-degree family relative. 3 Pursuit of research focused on Bsimplex[ and Bmultiplex[ families (i.e., with only one or with more than one sibling affected) has led to insights on potential modes and risks of inheritance, necessitating a unified genetic theory for