The present invention provides mammalian cell expression vectors that impart to mammalian host cells an ability to produce high levels of foreign gene-derived proteins. A ubiquitously acting chromatin opening element (UCOE) is introduced into an expression vector that has a plasmid DNA integrated into a transcriptional hot spot on the chromosome of a dihydrofolate reductase gene-deficient host cell so that it allows for selection of strains that grow in hypoxanthine-thymidine (hereinafter denoted as HT)-free medium, whereby transformants will produce a protein of interest in increased amounts.