Mammalien expression vector which contains a polylinker from Sac I to Sma I. The CMV promoter, which can be used to express the cloned cDNA, is followed by the hGH (human growth hormone) polyA signal located downstream of the insert. The ColE1 ori is the bacterial origin of replication, the SV40ori allows for replication in mammalian cells and the f1 ori is the filamentous phage origin of replication, which allows for the recovery of single-stranded plasmids. The ampicillin resistant gene confers the selection of the plasmid in E. coli. T7 RNA polymerase can be used for generating transcripts of the cDNA by in vitro transcription. However, the pCMVXL4 vector has a second but opposing T7 site (with a one-base substitution on the 3’ end) located between ColE1 ori and SV40 ori. Inserts (cDNA) in this vector must first be released by digesting with Sac I and Sma I before the in vitro transcription reaction.