Subjects with premutation alleles, i.e., repeat length between 55 and 200 will be excluded. Subjects with IQ less than 90 or subnormal language skills will be excluded. Subjects on psychotropic medication will be excluded from the study. Subjects who have received radiation doses for research purposes exceeding 3.5 rem (whole body effective dose) in the previous 12 months and subjects for whom MRI is contraindicated will be excluded. Subjects in whom sedation is contraindicated will be excluded from Part II. Subjects on medications that interfere with blood coagulation will be excluded (see Appendix 1). Subjects with any previous history of psychiatric or neurological disease, as assessed by the Structured Clinical Interview for DSM-IV (SCID), will be excluded. In addition, subjects with respiratory illnesses or cardiovascular diseases will be excluded from Part II as there might be increased risk of complications with sedation/anesthesia. Subjects positive for HIV will be excluded from the study. Subjects with positive results on the urine drug screen will be excluded.