Cosmid: The type of artificially constructed vector taken in use for cloning the 35-45 kb of DNA. These are plasmids carrying the phage l cos site (which permits packaging into l capsids), an origin of the replication and an antibiotic resistance gene. The plasmid of 40 kb is quite difficult to put into bacteria, but it can replicate once there. The cosmids, however, have the cos site, and hence can be packaged into l phage heads (the reaction which can be performed in vitro) to permit efficient introduction into the bacteria.