Fragile X syndrome (FXS) is a problem of the X chromosome. It is a condition that is inherited from the parents. FXS is the most common cause of inherited intellectual disability.

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FXS is caused by problems with the FMR1 gene. These problems prevent fragile X mental retardation protein (FMRP) from developing. The protein helps to make connections in the brain. Without the protein, certain connections in the brain cannot be made. This causes developmental problems and prevents children from fully developing some higher cognitive functions.