An expression vector was specifically designed for use in Chinese hamster ovary (CHO) cells to enhance the level of protein production in a transient expression system. Two key components that can increase protein production transiently are the promoter used to drive recombinant gene expression and the template copy number. In this study the modified and metal-inducible metallothionein (M2.6) promoter was shown to be superior to the human cytomegalovirus (CMV) and to the simian virus SV40 promoters. Plasmid replication was achieved using the Polyoma (Py) virus origin of replication (PyOri) and the Py Large T antigen (PyLT). An expression vector containing Py elements was shown to replicate extensively in CHO cells. The combination of the metal-inducible M2.6 promoter and episomal replication of the expression vector, named pPyOriLT resulted in elevated levels of transgene expression following transient transfection of CHO cells (c) 200S Society of Chemical Industry.