Because many different disorders can result in autism, this question is complex. Certainly, disorders such as the fragile X syndrome and tuberous sclerosis, which are both associated with autism, are inherited. There are many families with more than one child with autism where the autism is not clearly due to another cause. Recent studies have found that the gene for at least one kind of familial autism may be on chromosome 13. In some families, autism seems to be passed from generation to generation. In other families, autism is not found in prior generations, but affects multiple siblings (brothers or sisters). The results of this research make it likely that at least one "autism gene" will eventually be found. However, the majority of individuals with autism do NOT have a strong family history, which supports the premise that environmental or a combination of environmental and genetic factors contribute to the development of autism. In this context, environmental is meant to indicate any nongenetic factor, including infections, toxins, nutrition, or others.