A  normal  human  X  chromosome  (left)  contrasted  with  a fragile X
   chromosome (right), which is prone to breakage in culture.

   The syndrome is associated with excessive repeats of the trinucleotide
   sequence  CGG  in the FMR1 gene, which inactivates this gene. The FMR1
   gene  codes  for  an  RNA-binding  protein;  absence  of  this protein
   presumably affects brain development.