Complexity arises due to interactions among multiple genes, the environment, and epigenetic factors which do not change DNA sequencing but are heritable and influence gene expression. Many genes have been associated with autism through sequencing the genomes of affected individuals and their parents. However, most of the mutations that increase autism risk have not been identified. Typically, autism cannot be traced to a Mendelian (single-gene) mutation or to a single chromosome abnormality, and none of the genetic syndromes associated with ASD have been shown to selectively cause ASD. Numerous candidate genes have been located, with only small effects attributable to any particular gene. Most loci individually explain less than 1% of cases of autism. , it appeared that between 74% and 93% of ASD risk is heritable. After an older child is diagnosed with ASD, 7–20% of subsequent children are likely to be as well. If parents have one autistic child, they have a 2% to 8% chance of having a second child who is also autistic. If the autistic child is an identical twin the other will be affected 36 to 95 percent of the time. If they are fraternal twins the other will only be affected up to 31 percent of the time. The large number of autistic individuals with unaffected family members may result from spontaneous structural variation, such as deletions, duplications or inversions in genetic material during meiosis. Hence, a substantial fraction of autism cases may be traceable to genetic causes that are highly heritable but not inherited: that is, the mutation that causes the autism is not present in the parental genome. , understanding of genetic risk factors had shifted from a focus on a few alleles to an understanding that genetic involvement in ASD is probably diffuse, depending on a large number of variants, some of which are common and have a small effect, and some of which are rare and have a large effect. The most common gene disrupted with large effect rare variants appeared to be CHD8, but less than 0.5% of autistic people have such a mutation. The gene CHD8 encodes the protein chromodomain helicase DNA binding protein 8, which is a chromatin regulator enzyme that is essential during fetal development, CHD8 is an ATP dependent enzyme. The protein contains an Snf2 helicase domain that is responsible for the hydrolysis of ATP to ADP. CHD8 encodes for a DNA helicase that