The pCI and pSI Mammalian Expression Vectors are designed to promote constitutive expression of cloned DNA inserts in mammalian cells. The major difference between the two vectors is the enhancer/promoter region controlling expression of the inserted gene. The pSI Expression Vector contains the simian virus 40 (SV40) enhancer and early promoter region, whereas the pCI Expression Vector contains the human cytomegalovirus (CMV) major immediate-early gene enhancer/promoter region.