• miR-9 exhibits a developmental stage-specific expression pattern in the mouse cerebral cortex, with high levels during early neurogenesis and gradual decline thereafter.
• miR-9 promotes neuronal and oligodendrocyte differentiation while suppressing astrocyte differentiation in neural stem cells derived from the ventricular and subventricular zones.
• Gain- and loss-of-function experiments demonstrate that miR-9 is a critical regulator of trilineage differentiation balance in neural stem cells.
• These findings provide new insights into the molecular mechanisms underlying cortical development and potential therapeutic targets for neurodegenerative diseases.