Fragile X Syndrome is a common inherited cause of mental retardation that results from loss or mutation of the Fragile X Mental Retardation Protein (FMRP). Here we show that the mRNA of the basic helix-loop-helix transcription factor, human Achaete-Scute Homologue-1 (hASH1; ASCL1),is a new target of FMRP. hASH1 is required for normal development of the nervous system and has been implicated in the formation of neuroendocrine tumors. Using a double-immunofluorescent staining technique we detected an overlapping pattern of both proteins in the hippocampal formation, temporal cortex, subventricular zone and cerebellum of newborn rats. Forced expression of FMRP significantly increased hASH1 protein, but not its mRNA, in cultured HEK293 cells. On the contrary, siRNA mediated FMR1 knock-down in Kelly neuroblastoma cells significantly reduced hASH1 protein level. The positive correlation between FMRP and hASH1 proteins was further observed in primary hippocampal neurons. Sucrose density gradient centrifugation revealed that hASH1 transcripts were translocated into a translationally active polysomal fraction in response to increased FMRP expression. Luciferase reporter gene assays indicated that the influence of FMRP was mediated by interaction with the 5'-untranslated region (UTR) of hASH1 mRNA. The responsible cis-element was mapped by UV-cross-linking experiments and reporter mutagenesis assays to a (U)₁₀-sequence. In conclusion, hASH1 is a novel downstream target of FMRP. Improved translation efficiency of hASH1 mRNA by FMRP may represent an important switch in neuronal differentiation.