MicroRNAs are involved in different cancer-related processes. MicroRNA-21 
(miR-21), as an oncomiR, is overexpressed in all kinds of tumors and the role of 
miR-21 in carcinogenesis is elucidated in many cancers gradually. However, the 
function of miR-21 in osteosarcoma is still unclear. In our study, we found that 
miR-21 was significantly overexpressed in osteosarcoma tissues. More 
importantly, we confirmed that knockdown of miR-21 greatly decreased cell 
invasion and migration of MG-63. Furthermore, we identified that RECK 
(reversion-inducing-cysteine-rich protein with kazal motifs), a tumor suppressor 
gene, was a direct target of miR-21. Finally, the expression of RECK protein 
negatively correlated with the expression of miR-21 in human osteosarcoma 
tissues, indicating the potential regulation of RECK by miR-21. Our results 
suggest that miR-21 expression has a key role in regulating cellular processes 
in osteosarcoma, likely through regulating RECK and may serve as a therapeutic 
target.