A recent phase 2 clinical trial assessing BPN14770 (now Zatolmilast), a first-in-class phosphodiesterase 4D (PDE4D) inhibitor, in adult males with FXS demonstrated cognitive improvements on the performance-based NIH Toolbox Cognitive Battery and in caregiver reports of language and daily functioning (Berry-Kravis et al. 2021). This evidence concerns the gene PDE4D and fragile X syndrome.