One of a series of vectors (ATCC 87671 - 87674) that allows simultaneous, high-level expression of multiple cDNA sequences in mammalian cells. A kit of all four vectors is also available (ATCC 87678). Each cDNA inserted into one of this series of vectors is regulated by an identical transcription unit, but each vector permits different selection to be applied to different subunits. Host: mammalian cells, Escherichia coli