An expression vector is a type of plasmid used to introduce a gene into a target cell so that it is expressed by the cell's transcription and translation machinery. Expression vectors contain regulatory sequences that act as enhancers and promoters to efficiently transcribe the gene. They also require sequences that encode a polyadenylation tail, minimal untranslated regions, and a Kozak sequence to optimize mRNA production and translation. Plasmid vectors are commonly used genetic engineering tools that contain antibiotic resistance genes and an origin of replication allowing them to be replicated in bacteria and extracted for protein production. The pUC and pBR322 plasmids are examples of commonly used cloning vectors.