There is no known single cause of ASD, but it is generally accepted that it is caused by abnormalities in brain structure or function. For example, brain scans show differences in the shape and structure of the brains of children with autism. Researchers are investigating a number of theories, including the link between heredity, genetics, and medical problems such as hypoxia, maternal rubella, tuberous sclerosis, or epilepsy. In many families, there appears to be a pattern of autism or related disabilities, further supporting a genetic basis to the disorder. While no one gene has been identified as causing autism, researchers are searching for irregular segments of genetic code that children with autism may have inherited. It also appears that some children are born with a susceptibility to autism, but researchers have not yet identified a single "trigger" that causes autism to develop. Other researchers are investigating the possibility that under certain conditions, a cluster of unstable genes may interfere with brain development, resulting in autism. Still other researchers are investigating problems during pregnancy or delivery as well as environmental factors such as viral infections, metabolic imbalances, and exposure to environmental chemicals.