Cosmids - Cosmids are plasmids with "cos" site. Cosmids are a type of hybrid vectors that replicate like a plasmid but can be packaged in vitro into λ phage coats. A typical cosmid has replication functions, unique restriction endonuclease sites and selective markers (lacZ, antibiotic resistance contributed by plasmid DNA, combined with a λ DNA segment that includes the joined cohesive ends (cos site).

Cosmids provide an efficient means of cloning large pieces of foreign DNA. Because of their capacity for large fragments of DNA, cosmids are particularly attractive vectors for constructing libraries of eukaryotic genome fragments.

When transfecting the cosmid, fake dummy phage particles are mixed as this adding enhances the transfection efficiency.

Between 1 x 104 and 5 x 105 colonies of transformed cells are obtained per μg of insert DNA used. Because of the presence of plasmid elements, the recombinant transformants can be selected by using blue white selection.