This study is for boys aged 9 to <18 years with Fragile X Syndrome (FXS), a genetic condition that causes learning and behavioral challenges. It has two parts: Part 1 is open-label, meaning everyone knows the treatment, and checks how a medicine called BPN14770 moves through the body in doses of 25 mg and 50 mg. Part 2 is double-blind, meaning neither doctors nor patients know who gets the medicine or a harmless substitute called a placebo.

• The study requires regular clinic visits. Part 1 is already complete, and Part 2 is ongoing.
• All participants must be able to swallow capsules and weigh at least 75 pounds for the 50 mg dose.
• The patient and caregiver must commit to stable medication and treatment routines during the study.

To join, boys must have genetic proof of FXS and be on stable doses of allowed medications. Certain health issues, like heart or liver problems, might prevent participation. The study aims to understand BPN14770's effects and safety in treating FXS. Participants need to agree to attend clinic visits regularly and follow study guidelines closely.