Fragile X syndrome (FraX), a common inherited mental retardation and autism disorder, is caused by the loss of FMRP, an mRNA-binding protein that regulates mRNA stability and translation. Individuals with FraX have immature neuronal processes and decreased plasticity of mature synapses. So is this a disease of development, neuronal plasticity or both? On p. 1547, Tessier and Broadie reveal a prominent role for FMRP in activity-dependent neural circuit refinement during brain development in Drosophila. They show that in the fly FraX model, brain RNA and protein levels are increased during late brain development and during early-use refinement, a period of activity-dependent process pruning. FRMP expression normally peaks during this pruning period, they report, and is positively controlled by sensory input activity. Most importantly, FRMP expression is required for activity-dependent pruning during neural circuit refinement in the Mushroom Body, the brain region where learning and memory are consolidated. Together, these results reveal a critical late development role for FRMP and suggest that FraX is primarily a developmental disease.