Multiple twin and family studies have been conducted in order to observe any genetic influence in diagnosing ASD. The chance of both twins having ASD was significantly higher in identical twins than fraternal twins, concluding that ASD is heritable. A reoccurring finding is that "de novo" (new mutation) copy number variants are a primary cause of ASD - they alter synaptic functions; germ line mutations can produce "de novo" CNVs. These mutations can only be passed on to offspring; this explains the phenomenon that occurs when the child has symptoms of ASD, but the parents have no symptoms or history of ASD. "De novo" variants differ from person to person, i.e one variant can cause ASD in one person, whereas another person would need multiple variants to cause the same disorder. Loss of function variants occur in 16-18% of ASD diagnoses, which is nearly double the normal population. These loss of function variants reduce function in the protein neurexin, which connects neurons at the synapse and is important for neurological development; deletion mutations of neurexin are also very common in people with autism, as well as other neurological disorders like schizophrenia, bipolar disorder, and ADHD.