Single Dose Pharmacokinetic (PK) Study

Inclusion Criteria: 1. Adolescents with confirmed clinical and previous molecular diagnosis of FXS or AS, age between 13 to 17 years, inclusive. 2. Subjects must assent to participation in the study (if appropriate), have a parent or legal guardian/representative capable of providing informed consent on behalf of the subject, and commit to participate in all assessments described in the protocol. 3. Subjects must be receiving a stable dose of concomitant medications 4. Subjects should be able to complete study assessments. 5. Subjects who are non-sterile must agree to either remain completely abstinent or to use two effective contraceptive methods from screening until 7 days after the last dose of study treatment. 6. Subjects must have a parent or other reliable caregiver who agrees to accompany the subject at all study visits and provide information about the subject as required by the study protocol, and ensure compliance with the protocol. Exclusion Criteria: 1. Inability to swallow a capsule. 2. Poorly controlled seizures 3. Clinically significant abnormal ECG at the time of screening. 4. Positive result on serum or urine pregnancy test for women of child-bearing potential (have experienced menarche) who are not using a dual method of contraception (e.g., condoms plus oral contraceptives), with abstinence being an accepted method. 5. Allergy to gaboxadol or any excipients 6. Concomitant cardiovascular, respiratory, liver, renal, or hematologic diseases of a degree that would limit participation in the study 7. History of suicidal behavior or considered a high suicidal risk by the investigator. 8. Any medical, psychological, social disorder(s), or other conditions - including seizure disorder