The heritability of autism is the proportion of differences in expression of autism that can be explained by genetic variation; if the heritability of a condition is high, then the condition is considered to be primarily genetic. Autism has a strong genetic basis, although the genetics of autism are complex and it is unclear whether autism spectrum disorder (ASD) is explained more by multigene interactions or by rare mutations with major effects. Early studies of twins estimated the heritability of autism to be more than 90%; in other words, that 90% of the differences between autistic and non-autistic individuals are due to genetic effects. This however may be an overestimate: new twin data and models with structural genetic variation are needed. When only one identical twin is autistic, the other often has learning or social disabilities. For adult siblings, the likelihood of having one or more features of the broader autism phenotype might be as high as 30%, much higher than the likelihood in controls. Genetic linkage analysis has been inconclusive; many association analyses have had inadequate power. For each autistic individual, mutations in more than one gene may be implicated. Mutations in different sets of genes may be involved in different autistic individuals. There may be significant interactions among mutations in several genes, or between the environment and mutated genes. By identifying genetic markers inherited with autism in family studies, numerous candidate genes have been located, most of which encode proteins involved in neural development and function. However, for most of the candidate genes, the actual mutations that increase the likelihood for autism have not been identified. Typically, autism cannot be traced to a Mendelian (single-gene) mutation or to single chromosome abnormalities such as fragile X syndrome or 22q13 deletion syndrome. The large number of autistic individuals with unaffected family members may result from copy number variations (CNVs)—spontaneous alterations in the genetic material during meiosis that delete or duplicate genetic material. Sporadic (non-inherited) cases have been examined to identify candidate genetic loci involved in autism. A substantial fraction of autism may be highly heritable but not inherited: that is, the mutation that causes the autism is not present in the parental genome. Although the fraction of autism traceable to a genetic cause may grow to 30–40% as the resolution of array CGH improves, several results in this area have been described incautiously, possibly misleading the public into thinking that