The pMEI-5 vectors are high expression level retroviral vectors that enable gene transfer into mammalian cells. Following vector transfection into an appropriate packaging cell line, pMEI-5 facilitates transient or stable expression of products containing the psi virus packaging signal as well as the target gene. While the mammalian expression vector pMEI-5 possesses the LTR and psi viral elements, it does not carry the genes necessary for particle formation and replication, namely gag, pol and env. Because the pMEI-5 vector contains an intron with high splicing capacity, this enables a high rate of transcriptional efficiency, with a 2-8 fold higher gene expression level compared with pDON-Al-2 vectors. The pMEI-5 Neo vector offers the neomycin selectable marker for stable product expression.