A method has been devised for increasing the copy number of a gene (or genes) cloned into a plasmid while minimizing the size of the plasmid. If n copies of a transcriptional unit are cloned, including the promoter, coding region and terminator, the size of the plasmid will increase by n times the total size of the unit. However, if we borrow the concept of polycistronic operon and sandwich n structural genes, each with its own ribosome binding-site, between a promoter and a transcription terminator, there will be a space saving equivalent to n-1 promoters and n-1 transcription terminators. We have constructed plasmids in which an E. coli lipoprotein promoter is followed by 1 to 4 human leukocyte interferon genes and a transcription terminator. The applications of this method in genetic engineering are discussed.