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Cancer Susceptibility Candidate 9 (CASC9) is a novel gene generating long non-coding RNA (lncRNA) with oncogenic potential that was first identified in esophageal cancer. In this study, we have found that CASC9 was overexpressed in gastric cancer (GC) compared to normal gastric tissue. A higher expression level was associated with aggressive pathological characteristics, including deep invasion, poor differentiation and lymph node metastases. In two gastric cancer cell lines, BGC823 and SGC7901, CASC9 were both overexpressed compared to that of normal gastric epithelial cell (GES-1). Moreover, the expression of CASC9 was even higher in BGC823/DR and SGC7901/DR cells that are resistant to paclitaxel or adriamycin. CASC9 knockdown inhibited proliferation and promoted cell apoptosis In BGC823/DR and SGC7901/DR cells. The invasion potential was also significantly inhibited measured by Transwell assay. In addition, CASC9 knockdown in BGC823/DR and SGC7901/DR cells restored chemosensitivity to paclitaxel and adriamycin. This was associated with decreased expression of multidrug resistance 1 (MDR1) protein. Taken together, our data suggest that expression of lncRNA CASC9 correlated with aggressive pathological characteristics of GC, it may serve as a potential oncogene to regulate proliferation, invasion, and chemoresistance of GC cells.