Fragile X syndrome, the most common form of inherited mental retardation, is caused by the absence of the fragile X mental retardation protein FMRP.The RNA-binding FMRP represses translation of the microtubule-associated protein 1B (MAP1B) during synaptogenesis in the brain of the neonatal mouse.However, the effect of FMRP on microtubules remains unclear.Mounting evidence shows that the structure and function of FMRP are well conserved across species from Drosophila to human.From a genetic screen, we identified spastin as a dominant suppressor of rough eye caused by dfmr1 over-expression.spastin encodes a microtubule-severing protein and its mutations cause neurodegenerative hereditary spastic paraplegia.Epistatic and biochemical analysis revealed that dfmrl acts at upstream of or in parallel with spastin in multiple processes, including synapse development, locomotive behaviour and microtubule network formation.Immunostaining showed that both loss-and gain-of-function mutations ofdfmrl result in an apparently altered microtubule network.Western analysis revealed that the levels of α-tubulin and acetylated microtubules remained normal in dfinr1 mutants but increased significantly when dfmrl was over-expressed.To examine the consequence of the aberrant microtubules in dfmr1 mutants, we analyzed the microtubule-dependent mitochondrial transport and found that the number of mitochondria and the flux of mitochondrial transport were negatively regulated by dfmr1.These results demonstrate that dFMRP has a crucial role in controlling microtubule formation and mitochondrial transport.Thus, defective microtubules and abnormal mitochondrial transport might account for, at least partially, the pathogenesis of fragile X mental retardation.