Inclusion Criteria:  1. Patient is male adolescent aged 12 to < 18 years. 2. Patient has FXS with a molecular genetic confirmation of the full fragile X mental retardation-1 (FMR1) mutation (≥ 200 CGG repetitions).  3. Current treatment with ≤ 3 prescribed psychotropic medications. Anti-epileptic medications are permitted and are not counted as psychotropic medications if they are used for treatment of seizures. Anti-epileptics for other indications, such as the treatment of mood disorders, count towards the limit of permitted medications.  4. Permitted concomitant psychotropic medications must be at a stable dose and dosing regimen for at least 4 weeks before screening and must remain stable during the period between screening and the commencement of study drug.  5. Anti-epileptic medications must be at a stable dose and dosing regimen for 12 weeks before screening and must remain stable during the period between screening and the commencement of study drug.  6. Patients with a history of seizure disorder who are currently receiving treatment with anti-epileptics must have been seizure-free for 3 months before screening or must be seizure-free for 2 years if not currently receiving anti-epileptics.  7. Behavioral and other non-pharmacological treatments/interventions must be stable for 4 weeks before screening and must remain stable during the period between screening and the commencement of study drug, and throughout the study. Minor changes in hours or times of therapy that are not considered clinically significant will not be exclusionary. Changes in therapies provided through a school program, due to school vacations, are allowed.  8. Patient must be willing to practice barrier methods of contraception while on study, if sexually active. Abstinence is also considered a reasonable form of birth control in this study population.  9. Patient has a parent(s), legal authorized guardian(s), or consistent caregiver(s).  10. Patient and caregiver are able to attend the clinic regularly and reliably. 11. Patient's parent(s), legal authorized guardian(s), or consistent caregiver(s) is able to understand and sign an informed consent form to participate in the study.  12. Patient must provide assent for participation in the study if the patient has the cognitive ability to provide assent.  Exclusion Criteria:  Diagnosis and main criteria for inclusion:  The eligibility criteria are the same for all parts of the study except for Part 1 (PK), where patients must be able to swallow capsules and must weigh at least 75 lbs (34 kg) to receive the 50 mg dose.  Patient Inclusion Criteria  Patient is male adolescent aged 12 to < 18 years. Patient has FXS with a molecular genetic confirmation of the full fragile X mental retardation-1 (FMR1) mutation (≥ 200 CGG repetitions). Current treatment with ≤ 3 prescribed psychotropic medications. Anti-epileptic medications are permitted and are not counted as psychotropic medications if they are used for treatment of seizures. Anti-epileptics for other indications, such as the treatment of mood disorders, count towards the limit of permitted medications. Permitted concomitant psychotropic medications must be at a stable dose and dosing regimen for at least 4 weeks before screening and must remain stable during the period between screening and the commencement of study drug. Anti-epileptic medications must be at a stable dose and dosing regimen for 12 weeks before screening and must remain stable during the period between screening and the commencement of study drug. Patients with a history of seizure disorder who are currently receiving treatment with anti-epileptics must have been seizure-free for 3 months before screening or must be seizure-free for 2 years if not currently receiving anti-epileptics. Behavioral and other non-pharmacological treatments/interventions must be stable for 4 weeks before screening and must remain stable during the period between screening and the commencement of study drug, and throughout the study. Minor changes in hours or times of therapy that are not considered clinically significant will not be exclusionary. Changes in therapies provided through a school program, due to school vacations, are allowed. Patient must be willing to practice barrier methods of contraception while on study, if sexually active. Abstinence is also considered a reasonable form of birth control in this study population. Patient has a parent(s), legal authorized guardian(s), or consistent caregiver(s). Patient and caregiver are able to attend the clinic regularly and reliably. Patient's parent(s), legal authorized guardian(s), or consistent caregiver(s) is able to understand and sign an informed consent form to participate in the study. Patient must provide assent for participation in the study if the patient has the cognitive ability to provide assent. To participate in the Part 1 PK only: patients must be able to swallow capsules.  Patient Exclusion Criteria  Inability to successfully complete the NIH-TCB picture vocabulary and oral reading assessments at screening and baseline for Part 2 (DB). Patient must be able to complete these assessments at baseline to be randomized into Part 2; care should be taken that a patient enrolled into Part 1 (PK) possesses this ability if their desire is to continue to Parts 2 and 3. The ability to complete the NIH-TBC oral reading and picture vocabulary subtest at baseline is defined as the ability to complete both subtests, with (1) confirmation from the clinician administering that the test administrations are valid (noted on the administration form), and (2) generation of valid test scores for each test.  History of, or current cardiovascular, renal, hepatic, respiratory, gastrointestinal, psychiatric, neurologic, cerebrovascular, or other systemic disease that would place the patient at risk or potentially interfere with the interpretation of the safety, tolerability, or efficacy of the study drug.  • Common conditions such as mild hypertension, etc. are allowed per the PI's judgment as long as they are stable and controlled by medical therapy that is constant for at least 4 weeks before randomization.  Renal impairment, defined as serum creatinine > 1.25 × ULN at screening. Hepatic impairment, defined as alanine aminotransferase or aspartate aminotransferase elevation > 2 × ULN at screening. Note: liver function tests may be repeated after 1 week to evaluate return to acceptable limits; if liver function tests remain elevated, patient is ineligible to participate. Clinically significant abnormalities, in the PI's judgment, in safety laboratory tests, vital signs, or 12-lead ECG, as measured during screening. Positive COVID-19 test during screening. History of substance abuse within the past year, according to PI's assessment. Significant hearing or visual impairment that may affect the patient's ability to complete the test procedures. Concurrent major psychiatric condition (eg, major depressive disorder, schizophrenia, or bipolar disorder) as diagnosed by the PI. Patients with additional diagnosis of autism spectrum disorder or anxiety disorder will be allowed. Patient has active diseases that would interfere with participation, such as acquired immunodeficiency disorder, hepatitis C, hepatitis B, or tuberculosis. Patient is an immediate family member of anyone employed by the sponsor, PI, or study staff. Patient has weight < 60 lbs (27.2 kg) or a BMI greater than the 97th percentile for his age according to the Centers for Disease Control and Prevention (refer to Appendix 1). To participate in the Part 1 cohort receiving 50 mg dose, patient must weigh ≥ 75 lbs (34 kg).  Patient has participated in another clinical trial within the 30 days before screening.  -