A DNA cassette containing a trimerized SV40 polyadenylation signal which efficiently blocks spurious plasmid-initiated transcription.

A head-to-tail trimer of the SV40 Bcl I-Bam H1 DNA fragment, specifying polyadenylation of RNA transcripts, was cloned as a cassette flanked by multiple restriction sites. Insertion of the trimer into several expression vectors efficiently prevented spurious expression of reporter genes resulting from transcriptional initiation in prokaryotic plasmid sequences in transfected mammalian cells.