text: A cloning vector need not contain suitable elements for the expression of a cloned target gene, such as a promoter and ribosomal binding site (RBS),  many however do, and may then work as an expression vector.  The target DNA may be inserted into a site that is under the control of a particular promoter necessary for the expression of the target gene in the chosen host.   Where the promoter is present, the expression of the gene is preferably tightly controlled and inducible so that proteins are only produced when required.  Some commonly used promoters are the T7 and lac promoters.  The presence of a promoter is necessary when screening techniques such as blue-white selection are used.
  Cloning vectors without promoter and RBS for the cloned DNA sequence are sometimes used, for example when cloning genes whose products are toxic to E. coli cells.  Promoter and RBS for the cloned DNA sequence are also unnecessary when first making a genomic or cDNA library of clones since the cloned genes are normally subcloned into a more appropriate expression vector if their expression is required.
  Some vectors are designed for transcription only with no heterologous protein expressed, for example for in vitro mRNA production.  These vectors are called transcription vectors.  They may lack the sequences necessary for polyadenylation and termination, therefore may not be used for protein production.