Genomic libraries are made from total nuclear DNA of an organism. In making these libraries, it is important to cut the DNA into clonable size pieces as randomly as possible. Shearing or partial digestion with a frequently cutting restriction endonuclease are often used. Chromosome-specific libraries are made from the DNA of purified isolated chromosomes . There are thus liver, heart, leaf, root, etc.
Normalization eliminates many copies of highly abundant cDNAs.
Such expression is needed in screening libraries using antibodies or enzyme activities.