There could be a major breakthrough for millions suffering from two incurable mental disabilities -- autism and mental retardation.

Researchers at Massachusetts Institute of Technology reported that altering one gene significantly alleviated a wide range of abnormalities associated with "fragile X syndrome."

The disease affects hundreds of thousands of Americans.

It's a common inherited cause of mental retardation and autism.

The research was only conducted on laboratory mice. But scientists said changing the one gene, called the FMR1 gene, improved the brain development of mice, restored normal body growth and reduced seizures.

MIT researchers said experimental drugs could work the same way on humans.

Lead Researcher Dr. Mark Bear said it's a major step forward, and the first time there has been a biological rationale for a drug treatment for mental disorders.

"In the past, they've been viewed as really hopeless cases and the only strategy available was behavioral therapy," Bear said. "Now that possibility of a pharmacological treatment that could alter the course of disease which is I think is very exciting, that's what our work's doing, and that it's possible to correct the course or even reverse it with treatments that begin later in life."

Bear, a professor of neuroscience and director of MIT's Picower Institute for Learning and Memory, said it could be a treatment that you could start later in life, not in childhood, and still benefit.

They're waiting for Food and Drug Administration approval to start phase one of clinical trials on humans.

Bear said that could be as early as next year.

The studies were funded by the Howard Hughes Medical Institute, National Institutes of Health and National Institutes of Mental Health.