This is a single dose, placebo-controlled study. Male subjects aged 18 to 40 years (inclusive) with a diagnosis of FXS. Eligible subjects may enroll in this study comprised of two in home and two in clinic visits each 14 days apart, for a total of four visits. Subjects will be given single dose gaboxadol (10 mg) or matched placebo at each of these visits to take orally. Thus, all enrolled subjects will receive placebo at home and in clinic and receive gaboxadol at home and in clinic in a blinded fashion.

Inclusion Criteria:

1. Subject consents to participate, or if they are not their own legal guardian, offers assent supported by legally authorized representative consent
2. Subject is willing and able to comply with the study procedures as specified in the protocol and to comply with the study drug administration. Caregiver also commits to the study requirements prior to any study-related procedures
3. Subject and caregiver are both able to understand the spoken national language clearly and caregiver can read and write to complete study assessments
4. Males age 18 to 40 years (inclusive)
5. Has FXS with molecular genetic confirmation of the full FMR1 mutation (>200 cysteine-guanine-guanine [CGG] repeats). May have been confirmed historically or at Screening
6. Is in general good health as deemed by the Investigator, determined by physical examination, medical history, and laboratory tests
7. If receiving serotonin-selective reuptake inhibitor (SSRI), serotonin-norepinephrine reuptake inhibitor (SNRI), or serotonin antagonist and reuptake inhibitor (SARI), is on a stable, well-tolerated dose for the previous 3 months with no further changes anticipated
8. Is not sexually active or can confirm at least one form of contraceptive

Exclusion Criteria:

1. Any chronic major medical comorbid condition deemed by the Investigator as presenting added risk to the subject, including but not limited to, refractory hypertension, kidney disease, or liver disease
2. Diagnosed with diabetes (Type 1 or II) or receiving any anti-diabetic medication
3. Unstable seizure disorder, defined by any seizure within 6 months prior to baseline visit and/or a change in any anti-convulsant drug dosing in the 60 days prior to study consent
4. Changes in psychotropic or anti-convulsant (where taken for reasons other than seizure control) drug treatment within 30 days prior to Screening
5. Significant changes in any educational, behavioral, and/or dietary interventions the month prior to Screening
6. Planned initiation of new, or modification of ongoing, interventions during the study
7. Unable or unwilling to take oral medication (whole capsule, despite assistance with a spoonful of applesauce, yoghurt, or equivalent liquid food)
8. Consumption of liver enzyme inducers or inhibitors including and not limited to foods, medicines, herbal remedies and supplements three days prior to any Visit. Foods or beverages containing CYP3A4/5 inhibitors (e.g., grapefruit, pomegranate, pomelo, and star fruit) should be avoided before taking study medication and for up to 1 hour post dose throughout the study
9. Has abnormal baseline laboratory assessments including, but not limited to, alanine aminotransferase (ALT) or aspartate aminotransferase (AST) or total bilirubin >1.5 × the upper limit of normal (ULN), serum creatinine >1.5 x ULN or other clinically relevant laboratory abnormality
10. Has a clinically significant heart rate or blood pressure (BP) at Screening as judged by the Investigator
11. Has received an investigational drug in any prior clinical study within 30 days or 5 half-lives (whichever is longer) prior to Screening

Quadruple masking (participant, care provider, investigator and outcomes assessor)