Cosmids: a type of plasmid gene-cloning vector that is packageable in vitro in bacteriophage lambda heads.

Evidence is presented that ColE1 hybrid plasmids carrying the cohesive-end site (cos) of lambda can be used as gene cloning vectors in conjunction with the lambda in vitro packaging system of Hohn and Murray [(1977) Proc. Natl. Acad. Sci. USA 74, 3259--3263]. Due to the requirement for a large DNA molecule for efficient packaging, there is a direct selection for hybrids carrying large sections of foreign DNA. The small vector plasmids do not contribute a large background in the transduced population, which is therefore markedly enriched for large hybrid plasmids (over 90%). The efficiency of the in vitro packaging system is on the order of 10(5) hybrid clones per microgram of foreign DNA for hybrids in the 20--30 million dalton range.