E.Coli Vectors - Bacteria are the hosts of choice for DNA cloning. Among them, E. coli occupies a prominent position since cloning and isolating DNA inserts for structural analysis is the easiest in this host. Therefore, the initial cloning experiments are generally carried out in E. coli.

The E. coli strain K12 is the most commonly used; it has several substrains, e.g., C600, RRI, HB101, etc., each of which has some specific features important in cloning. For example, the subs train RRI has, in addition to certain other features, the mutation hsdR, which inactivates the restriction enzyme endogenous to E.coli K12; this minimises the degradation of recombinant DNA introduced into it.