Fragile X syndrome.

The fragile X syndrome is characterized by mental handicap, facial dysmorphism and expression of a fragile site at Xq27.3. An expansion of a CGG repeat in the 5' end of the fragile X mental retardation 1 (FMR1) gene results in the absence of the encoded fragile X mental retardation protein, known to play an important role in RNA processing and probably the developmental maturation of brain neurons.