Washington ? A pill to ease a type of mental retardation? An experiment is under way to develop one, aimed at a genetic disorder that might unravel some of the mysteries of autism along the way.

Chances are you’ve never heard of the target — Fragile X syndrome — even though it’s the most common inherited form of intellectual impairment, estimated to affect almost 100,000 Americans. It’s also the most common cause of autism yet identified, as about a third of Fragile X-affected boys have autism.

Now a handful of drug makers are working to develop the first treatment for Fragile X, spurred by brain research that is making specialists rethink how they approach developmental disorders.