“So far we like what we see,” Dr. Santarelli said in his only characterization of their study.

One reason for the euphoria surrounding the Novartis trial is that it was seen as an especially difficult test of the drug’s effects. For ethical reasons, Novartis tested the drug only in adults. But the company and outside researchers believe that such compounds may prove most effective in young children, whose brains are far more likely to respond rapidly when barriers to learning are removed.

“This is perhaps the most promising therapeutic discovery ever for a gene-based behavioral disease,” said Dr. Edward M. Scolnick, former research chief at Merck and now director of the Stanley Center for Psychiatric Research at the Broad Institute at Harvard and the Massachusetts Institute of Technology.

Dr. Scolnick has not seen the results of the Novartis trial, but was told of them and concluded that if the drugs work in fragile X, “there’s nothing to say that they won’t work in some cases of broader autism-spectrum disorders.”

An Unlikely Beginning

The roots for the Novartis results began in 1982 when Stephen T. Warren, then a graduate student in genetics at Michigan State University, was looking for a job and something to research. A friend told him about fragile X and, with the same reflection he might use to pick a novel for a long flight, he decided that he wanted to find the gene that caused it.

“I had no idea how hard this would be,” Dr. Warren said. Nine years later, Dr. Warren, then at Emory University, was part of an international team that won a fierce competition by isolating the gene. The discovery was front-page news around the world, and experts predicted that widespread fetal testing and therapies were in the offing.

The predictions were premature because, like most of genetic research, discovering how the flawed gene caused disease was far harder than anticipated and required multiple leaps in neurology and biology. And even with those, much remains mysterious.

Fragile X is caused by a genetic stutter in which a portion of the gene gets repeated like a scratched album. With each subsequent generation, the number of repeats tends to rise. So if a mother has 10 repeats, her child might have 11 or 12. For reasons that are not well understood, however, this process of repeat amplification can suddenly go haywire. So mothers who have 55 or more repeats tend to have children with hundreds.