Fragile X syndrome (FraX, also FXS) is the most common known heritable cause of human intellectual disability. Though recent research has revealed much about the genetic and neurobiological bases of FraX, knowledge about specific and effective treatments for affected individuals is lacking. Based on information from both human and animal studies, one cause of intellectual disability in FraX may be related to deficits in a particular brain neurotransmitter system (the "cholinergic" system). Thus, this study will use a specific medication, donepezil, to augment the cholinergic system in adolescents affected by FraX. If found to be effective, the knowledge generated by this research may also be relevant to other developmental disorders that share common disease pathways with FraX.