MMP-9 is highly expressed in osteosarcoma [38, 39], in our study, MMP-9 expression was down-regulated after CD44 was knocked down, reflecting the promoting effect of CD44 on invasion and metastasis of osteosarcoma cells, which is consistent with the role of CD44 in other tumours [40, 41]. This evidence concerns the gene CD44 and osteosarcoma.