Fragile X syndrome is an X chromosome defect that causes mental retardation and a wide range of associated signs and symptoms.

This syndrome is the second most common identifiable cause of genetic mental retardation after Down syndrome. It was first identified by geneticist Herbert Lubs, who observed the chromosomal defect responsible for the syndrome in 1969.

Some people have chromosomes that when studied in a laboratory, have a tendency to "break" or "tear." The damage to these aberrant...