A 3-year-old girl, her mother, and maternal uncle had microcephaly and mental retardation. Their facial appearance is characterized by deep-set eyes, short philtrums, and a "beaked" nose. The mother and uncle live in an adult foster care facility because of mental retardation. The 3-year-old girl has a developmental quotient of 55. Mother has normal phenylalanine level and the child's chromosom...

Introduction Fragile X Syndrome (FXS) is the most common form of inherited mental retardation, caused by a trinucleotide repeat expansion resulting in silencing of the fragile X mental retardation 1 (FMR1) gene in humans. The array of disorders caused by mutations in this single gene includes mental retardation and autistic behaviors. There is much evidence that the protein product, fragile X m...