Fragile X syndrome is caused by a mutation on the FMR1 gene on the X chromosome. It is the most common inherited cause of intellectual disability. The mutation involves a trinucleotide CGG repeat that silences the FMR1 gene, preventing production of the FMRP protein needed for normal brain development. Affected individuals have intellectual disabilities, autism-like behaviors, anxiety, and other issues. Males typically have more severe symptoms than females. While there is no cure, early intervention and therapies can help manage symptoms.