There are believed to be as many as a thousand tied to autism, each one incredibly rare.

(NEWSER) – Big autism news: A number of gene mutations linked to the disease have been uncovered for the first time, announced a number of scientists in three papers published yesterday.
The sobering detail: These particular mutations are super rare, and are responsible for only a very small number of autism cases. The encouraging detail: The discovery gives scientists a foothold from which they can start growing their knowledge of autism's biological roots.
Though they've longed believed genetics are at play, there hasn't been agreement on how to proceed; this discovery changes that, reports the New York Times.