Libraries may be highly complex (as when cloning complete genomic DNA from an organism) or relatively simple (as when moving a previously cloned DNA fragment into a different plasmid), but it is almost always necessary to examine a number of different clones to be sure that the desired DNA construct is obtained.
Celotno populacijo posameznih klonov, ki jih dobimo v poskusu molekulskega kloniranja, pogosto imenujemo knjižnica DNA.