A key problem in multiple sclerosis (MS) is the diminished capacity for myelin repair. Although oligodendrocyte (OL) precursors can be seen at the lesion site, their ability to differentiate appears inhibited. MicroRNAs are key regulators of OL differentiation, and have been observed to be misregulated in MS lesions compared to healthy white matter. Thus, aberrant microRNA expression in MS lesions may disrupt the ability of incoming oligodendrocyte progenitor cells (OPC s) to differentiate. Specifically, a microRNA known as miR - 145 is downregulated as OPCs progress to OLs, but is found at unusually high levels in MS lesions. In this study, we investigated how misregulation of miR - 145 affects OL differentiation in vitro. Bioinformatic analysis revealed that putative targets of miR - 145 are significantly enriched for factors which promote actin cytoskeleton organization and myelination. An immortalized OL cell line was transduced with an inducible lentivirus to create stable lines that overexpress miR - 145. These stable lines were characterized while proliferating, early in differentiation and late in differentiation. Immunofluorescence was used to quantify changes in proliferation rate, apoptosis, branching ability and myelin gene expression. qPCR arrays were used to quantify changes in microRNA target expression levels between induced and uninduced cells. Two stable lines were created: ON - 145 - 1 and ON - 145 - 2, which upon induction, over - express miR - 145 ~33 - fold and ~11 - fold, respectively. When proliferating, no significant morphological differences nor target expression differences could be detected between induced and uninduced cells. Proliferation was significantly decreased in ON - 145 - 1 induced cells, but not in ON - 145 - 2. No changes in apoptosis frequency were detected. In contrast, during early and late differentiation, both induced cell lines showed significant morphological defects characterized by a reduction in both iii primary and secondary branching. Further, significant differences in branching ability were observed between induced cells of ON - 145 - 1 and ON - 145 - 2, suggesting a dose - dependent response to miR - 145 overexpression. Expression of MAG, a myelin marker, was also significantly lowered in induced cells of both cell lines. Finally, we found that multiple miR - 145 targets involved in promoting cytoskeletal organization and myelination were significantly decreased both early and late in differentiation. These results suggest that overexpression of miR - 145 during OL differentiation may disrupt actin organization and myelin gene expression required for successful process extension and subsequent myelinating ability. Thus, the increase in miR - 145 in MS lesions may be a significant contributing factor to the loss of myelin repair in MS lesions.