POU5F1B is a processed pseudogene and highly homologous to OCT4, which is crucial in the regulation of pluripotency during embryonic development of embryonic stem cell. Since recent reports have demonstrated that POU5F1B mRNA is expressed in several types of cancer tissue, their oncogenic role remains unclear. Here we further investigated that the biological function of POU5F1B. We found that POU5F1B, which is located side by side with MYC gene on 8q24, was frequently involved in MYC amplicons in gastric cancer. Gene copy number analysis revealed that POU5F1B and MYC genes were highly amplified in several gastric cancer cell lines. In addition, the POU5F1B mRNA expression levels were certainly upregulated using PCR and sequencing analysis. Then, we hypothesized that gene amplification of POU5F1B mediates some biological roles in cancer cells. We introduced POU5F1B gene into SNU-16 or TU-KATO III gastric cancer cell lines and established stable cell lines SNU-16/GFP, /POU5F1B, TU-KATOIII/GFP and /POU5F1B. Overexpression of POU5F1B clearly decreased cellular proliferation and inhibited colony forming ability. Interestingly, we found that the increased gene copy number of POU5F1B is a significant poor prognostic factor in 180 cases of gastric cancer. We are now under investigation for its oncogenic properties, gene expression changes by microarray analysis and chemoresistance using these stable POU5F1B transfectants.