The fragile X syndrome is characterized by mental handicap, facial dys
morphism 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 retardati
on protein, known to play an important role in RNA processing and prob
ably the developmental maturation of brain neurons.