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In order to investigate the expression of human cellular glutathione peroxidase (GPx), we mutated the gene encoding GPx by deleting either the 5′ or 3′ untranslated region (utr), subcloned the deleted fragments into plasmid pSVL followed by transfection into COS-7 cells and measured the amount of GPx expressed. When the 5′ utr of the gene was deleted, GPx was not expressed. However, the deletion of the 3′ utr resulted in some expression of GPx. Deletion of the poly A region of the GPx gene resulted in the expression of GPx but the level was lower than that of the full-length cGPx. The complete deletion of the 3′ utr resulted in a half of the expression of the poly A deletion mutant. Thus, the expression of GPx increased according to the length of the 3′ utr. These results suggest that the GPx gene carrying one SECIS on 5′ utr (FEBS Lett. 312(1992)10-14) is essential for GPx expression. SECIS on 3′ utr might not play a key role of GPx expression. Expression of GPx by COS-7 cells was not observed when a plasmid harboring an antisense gene was transfected.