Objective To assess the role of microRNA-9 in bone marrow mesenchymal stem cells differentiating into neurons and research the role of gene modification in spinal cord injury treatment.Methods Adherent culture was used to isolate and culture rat bone marrow mesenchymal stem cells (MSCs); microRNA-9-1 lentiviral vector was constructed.Acute spinal cord injury (SCI) models were established in 84 adult SD rats at T10/11 level according to the improved Allen's method; then,they were randomly divided into control group,MSCs group and miRNA group (n=28).One week after SCI,the rats of MSCs group were treated with MSCs implantation,and the rats of miRNA group were treated with MSCs-transfected microRNA-9-1 lentiviral vector; while rats of the control group only received the same amount of physical saline in the same region.The neurological functions were evaluated by using Basso-Beattie-Bresnahan (BBB) scale 1 and 3 days,and 1,2,4,6,8 and 12 weeks after SCI.The immunoreactivity of neuro filament 200 (NF-200) and glial fibrillary acidic protein (GFAP) was measured,and the percentage of positive response area was assayed and compared between groups.Results Four weeks after cell transplantation,statistical difference of BBB scale scores was noted between each two groups; the scores of miRNA group were significantly higher than those of the MSCs group and control group (P＜0.05).The immunohistochemistry staining indicated that the expression of NF-200 was significantly more intense and the expression of GFAP was significantly weaker in miRNA group than those of the other two groups (P＜0.05).Conclusion MicroRNA-9 may play an important role in bone marrow mesenchymal stem cells differentiating into neurons,and possess effects on repairing injured spinal cord and promoting functional recovery through promoting axonal regeneration and reducing the number of reactive glial cells in the spinal cord injury site.