FMR1 and fragile X syndrome: Fragile X syndrome is the most common inherited cause of ID, affecting approximately 1 in 4,000 males and 1 in 8,000 females, and is often associated with cognitive impairment, seizures, and anxiety. Fragile X syndrome results from a CGG short tandem repeat (STR) expansion in the FMR1 gene, resulting in altered gene expression.