The goal of this study was to determine whether a phosphodiesterase-4D (PDE4D) allosteric inhibitor (BPN14770) would improve cognitive function and behavioral outcomes in patients with fragile X syndrome (FXS). This phase 2 trial was a 24-week randomized, placebo-controlled, two-way crossover study in 30 adult male patients (age 18–41 years) with FXS. Participants received oral doses of BPN14770 25 mg twice daily or placebo. Primary outcomes were prespecified as safety and tolerability with secondary efficacy outcomes of cognitive performance, caregiver rating scales and physician rating scales (ClinicalTrials.gov identifier: 