DNA sequences in a typical E. coli expression vector. The gene to be expressed is inserted into one of the restriction sites in the polylinker, near the promoter (P), with the end of the gene encoding the N-terminus of the protein positioned closest to the promoter. The promoter allows efficient transcription of the inserted gene, and the transcription-termination sequence sometimes improves the amount and stability of the mRNA produced. The operator (O) is a sequence bound by a protein called a repressor, which normally blocks gene expression from the adjacent gene. The ribosome-binding site provides sequence signals for the efficient translation of the mRNA derived from the gene. The selectable marker allows the selection of cells containing the recombinant DNA.