A large study of almost a thousand autistic individuals for genetic variations that make them different from control individuals has found that Autism Spectrum Disorder has many different genetic causes: there isn't one single gene responsible for ASD, but a constellation of hundreds, each with the potential to affect the development of the brain and cause the symptoms of autism. They don't know exactly how each of these genes contributes to the disorder, but they have found that many of them are involved in growth and cell communication and the formation of synapses in the brain.
The bottom line is that there are many different ways to cause the symptoms of autism, and it's a mistake to try to pin it all on single, simple causes. Any hope for amelioration lies in understanding the general functional processes that are disrupted by mutations in various pathways.
[amelioration: the act of relieving ills and changing for the better]