The pCI Mammalian Expression Vector promotes constitutive expression of cloned DNA inserts in mammalian cells. The major difference between the pCI and pSI Mammalian Expression Vectors is the enhancer/promoter region controlling the expression of the inserted gene. The pCI Expression Vector contains the human cytomegalovirus (CMV) major immediate-early gene enhancer/promoter region. This vector can be used for both transient and stable expression of genes. For stable expression, the pCI Vector must be co-transfected with an expression vector containing a selectable gene for mammalian cells.