After the construction of recombinant lambda or cosmid libraries the total DNA is transferred into an appropriate "E. coli" host via a technique called in vitro packaging. The necessary packaging extracts are derived from "E. coli" cI857 lysogens (red- gam- Sam and Dam (head assembly) and Eam (tail assembly) respectively). These extracts will recognize and package the recombinant molecules "in vitro", generating either mature phage particles (lambda-based vectors) or recombinant plasmids contained in phage shells (cosmids). These differences are reflected in the different infection frequencies seen in favour of lambda-replacement vectors. This compensates for their slightly lower loading capacity. Phage libraries are also stored and screened more easily than cosmid libraries.